WoW is for casuals
The post by XI on Death and Taxes' site puts their disbanding down to people, not the "casualization of the game". However, recent guilds to disband such as Risen and Core have gone the other way -- pinning their problems on the advent of welfare epics, epic gems for all and the general accessibility of endgame-level loot to people who never step outside a 5-man. While this in itself isn't guild-killing, it certainly dampens morale to see someone in levelling greens using a better weapon than you, and if your entire raid force is thinking "Why am I wasting time here when I can get the same sort of loot from just doing Karazhan?" then your progress won't be great.
Of course, it isn't as simple as that. Simply seeing encounters that others can't, having a certain guild tag, raiding with certain people, boasting about Entropius dps -- these are all drivers for endgame raiders that aren't affected by loot. However, that feeling of being ahead of everyone else got negated somewhat when the Black Temple and Hyjal attunements were lifted, and that's also contributed to raiders not feeling "special" any more. Watching people wiping on Supremus can help you regain that feeling, though!
This game isn't about giving a small percentage of players the right to feel superior -- TBC has long been about accessibility, and for a lot of people who did things the hard way only to see nerf after nerf, attunement removal, badge and PvP epics as well as encounters that didn't really feel endgame-worthy, raiding has come up with more disappointments than pleasures. Naturally people who feel particularly strongly about one or all of these will drift away from raiding or the game in general, and if that's most of your guild, well -- we've seen what happens.
The downtime blues
Before the release of Sunwell, a lot of guilds spent months and months farming tier 6 content. This level of farming has been seen before while cutting-edge raiders waited for the next instance, but the sheer number of people stuck on the tier 6 plateau with nothing else to do meant that 'cutting-edge' applied to a lot of guilds and boredom was prevalent. Many raiders quickly got tired of this farming, and while some were prepared to stick it out and get that last item, or gear up those who helped them get geared, others decided they had better things to do with their time.
We've seen several people have important real life things to attend to during the learning and early farm periods in Black Temple, just as in other instances, then a couple of months down the line they suddenly have time for WoW again when -- coincidentally -- everything's on farm. Similarly, once some people had most of their gear, they surprisingly can't make raids... until the last few weeks before a new instance is released, when competition for their last few drops is non-existent too. It's only natural to get bored doing the same content for months, but not knowing from week to week who'll turn up, who'll burn out, who'll have more important things to do can really cause problems for a guild.
Firstly, if someone's dropped off the radar, you might need to recruit -- which will just cause problems if you let them come back. If people vanish with a lot of gear, that's unfortunate since it annoys the more dedicated raiders who wanted that gear. Loot isn't everything, but people still get pretty worked up about it. Having people who were MIA for months suddenly turn up and demand a raid spot also causes tension, especially if they gave a lot to the guild in the past. It's also tricky to figure out if someone is just trying to skip the hard bits and collect their epics, or if they genuinely have a RL commitment that gets in the way.
Towards the end of a long farm period, people do tend to treat raiding like a fairly mundane job. Log in at 7pm, push buttons, collect epics. None of the challenge we know and love is there; some of the fun, perhaps (but solely due to the people). With a tier 6 raid clearing tier 6 content, people can be half-asleep and underperforming and get away with it. In short, people forget what progress raiding is all about, and enjoy seeing their average ilvl get higher and higher. Then, suddenly:
New content is hard
No doubt people will disagree with this statement, but from the point of view of people who've been farming the same 14 bosses for half a year, Sunwell is hard. The encounters are well-designed and have a high level of personal accountability, something which was lacking from most of the previous bosses. People who have been half-asleep and dozed their way through glorified trash suddenly have to perform, and each guild's found themselves with a number of raiders who don't meet that mark, despite being adequate before.
So, this new instance has hit, and you've got dead weight you need to prune. You've got people who liked logging in to collect epics, but don't like having to farm consumables and wipe all night, so they mysteriously stop showing up. You've got people who could do a 3 day/week raid schedule but balk at 5. You've got raid leaders who are happy to run raids when they know the encounters inside-out, but who pass the buck when it comes to progress content.
What do you do? Suddenly, you're down to half the raiders you thought you had, key classes are missing and all the gear you got in Black Temple is sitting on characters who can't be bothered turning up -- or who aren't good enough to get raid spots. Recruiting is interesting, because plenty of other guilds are in the same situation -- some disband, and populate the recruitment 'market' with exactly the sort of t6-geared, experienced player you need, but of the wrong class. Or you have to take people in worse gear and spend valuable progress raid days gearing them up, to the dismay of the 15 or so 'core' members who never wanted to see the inside of those instances again.
If only you'd recruited back when everything was on farm. Oh, wait -- you did, but half those players got a rude awakening when stuff got hard, and they're the ones you have to replace now since you never really got to test them on farm content.
This is, perhaps, an over-exaggeration of what's happened in most guilds, but we've seen several entirely different guilds go through this "Sunwell shock" phase, and others go through exactly the same thing at the start of TBC. In others, the majority of the raid force has been eager to see new content and attrition due to the switch from farm to progress has been very low. Server firsts, PTR testing and a generally healthy attitude to recruitment and evaluation seem to be common in these lower-attrition guilds. This goes back to pride -- pride and firsts are a great motivator, and if people have something to gain beyond loot they won't desert at the first mention of wipe nights.
Leaving is easy
Paid character transfer has been a double-edged sword to recruiters. On servers where there aren't a great deal of 'feeder' guilds, picking up people at your gear level cross-realm can be a godsend and saves you an awful lot of time. Sometimes cross-server recruits don't work out, but for the most part they do, and having access to an entire region's worth of applicants (or a subset of them if you're PvE) is great.
However, it makes it easy for people to jump ship from your guild -- which may be best on server -- to one slightly further ahead elsewhere. We haven't seen many cross-server exits, but knowing some new recruits came from guilds that were in the process of dissolving, and that by recruiting them you probably helped kill that guild, is quite a conscience-stirrer.
The lack of PvE to PvP transfers and the obvious cross-faction barrier do help control the flow of raiders to some extent, but it's fairly easy to simply reroll these days, especially if you have a reputation and contacts. Thanks to badge epics, you can gear up very quickly with help, and if you join a guild that's farming tier 6 you'll be decked out in 5/8 in no time. Some people become quite invested in their mains, and this obviously isn't an option for them, but it's another valid path for those disillusioned with their faction, character or guild.
Atmosphere and language
One discussion that's sprung up in the wake of the Death and Taxes news is that of guild atmosphere changing over time -- some people rising to prominence and "spoiling the fun" for others, people using language that others found offensive, and whether said language was offensive in the first place. While etymology and idiom are fascinating subjects, what we can learn from this is that raiding isn't always the same experience, even if you're in the same guild with (mostly) the same people.
Recruiting -- a must, if you run into any of the problems described above -- naturally changes the flavour and character of a guild. While you may have common goals and ideals, you'll become a different group of people to the raid that killed Kel'Thuzad; to the group that killed Kael'Thas for the first time. The leadership changes as well as the member base, cliques develop and naturally the place changes over time. Sometimes guild members will stop, take stock and realise that the place they are now isn't the one they joined, nor is it where they want to be. That's natural enough, though one thing to be learned from some of the comments about Death and Taxes is that letting members play for power and abuse other members is likely to cause this sooner rather than later.
Trouble at the top
Especially during transition and progress periods, officers and guild leaders have it tough. Farm periods aren't totally without drama, but it all seems to happen at once during progress -- especially if you're cutting people who underperform out of raiding, or hastily trying to plug recruitment gaps caused by those who left. Add to that difference of opinions within a guild -- you'll always find people pushing for old content while others want to spend seven days a week in new -- and other overheads such as changing the DKP system, working out tactics and so forth, and soon it's hard to remember why officers are mad enough to hold the position.
Officer and GM burnout isn't rare by any means, and the downside to it is that there aren't always people ready to step into their shoes, so the remaining officers end up with more to do and thus more chance of burning out. There's nothing odd about officers who stop playing or who stop caring, but you have to be aware of things you can do if this happens, and ensure that communication between officers is good enough that there's someone who can take over if one burns out. Of course, this is a bit harder if it's the GM who burns out, but guilds have coped with this just fine -- just take your GM's orders with a pinch of salt when he suddenly becomes interested in making decisions after not raiding for a month.
So what can you do if an officer leaves? Keep an eye on your members to see who might be a good addition to the officer team in future; maybe test them out on offday or farm content runs. Get the officers who aren't raid leaders to lead some farm raids. Ensure the tasks that officer did are done by other people; you don't need a certain guild rank to do things like upload WWS or post recruitment notices. This all depends on how your guild runs itself and how your officer team operates, but losing one or more officers at a crucial time with no idea what to do about it can be crippling.
Towards the end of vanilla WoW, before TBC hit, we saw a lot of apathy hit raiders. Why should we spend our evenings toiling away and wiping on bosses in Naxxramas, when in a month or two we'll be replacing those hard-earned epics with level 60 greens? The same attitude is hitting people now, even though Wrath is a while away. Sunwell loot is supposed to last to early level 80 content, but that doesn't stop people wanting to spend more time getting their alts and bank balance ready for the expansion rather than making their main a little bit more awesome.
This will get more prevalent as Wrath approaches, but having raiders ready for the expansion is no bad thing. It simply becomes a matter of balancing time and ensuring people are still interested in the raid content as well as getting opportunities to do whatever else they feel they need to. How do you keep people interested? Ultimately, other people. Once things are on farm again, loot will come, but without a team who -- for the most part -- enjoy working and raiding together, you won't have a strong base for the expansion. Take the time now to build that up, and you'll be perfectly prepared.