Once you master the art of the 5-person PuG, the ultimate risk is a raid PuG. One-shot the instance, or spend the night wiping? You won't know until you try.
I used to run Hyjal PuG's in late Burning Crusade and got to be the person in charge of arranging healers on Anetheron, explaining where to die on Azgalor, and uttering a hollow laugh at suggestions on whether or not Archimonde was in the cards (answer: hell no). I wasn't around for my guild's Naxx run one of these past weeks, and a few guildies were interested in gearing up their alts, so we thought -- PuG a 25-man Naxx? Why not?
1. Sending an open invite for tanks, healers, and DPS into the trade channel provides a pretty instructive look at what people are most interested in playing and gearing right now.
My totally unscientific take on things; it's a lot easier to PuG a tank now than it was in BC, healers are still scarcer than hens' teeth (but this will get easier if they know your run has a good track record), and -- as predicted -- everyone and his brother has a Death Knight. I could easily have filled every non-healer slot with a Death Knight, and wound up running 3 Death Knight tanks and 3 Death Knight DPS. For the forseeable future, I think you can realistically expect the Vanquisher token to be a complete nightmare.
2. Trying to explain Heigan to new raiders apparently sounds like you're reciting The Decameron as translated from Esperanto into Farsi and back again.
I think a 20 minute, 27 second kill says it all, although I await the legion of commenters certain to pop up and say, "You think that's bad?"
Tip: send people here, don't pull until everyone's seen it, assign an experienced "dancer" a raid icon, and tell people to stick to this person like glue on phase 2. I made the mistake of assigning this icon to one of our DK off-tanks, who assured me he knew the fight. It turns out he really didn't, and I spent each phase 1 bawling, "Move now! OK forward! More! More! Stop right there! Great! OK, forward! Back a little bit! Stop right there!" until I went hoarse.
3. On that note, who's tanking for you still matters a lot.
Tanks invariably set the pace of the run, and if they've never seen a fight before, you'll wind up having to spend a lot more time before each boss pull explaining what they have to do. Try to run with at least one tank who's familiar with the fights, even if they're not the best-geared. They are still likely to outperform a better-geared tank who stares blankly when you ask them in which direction they'll be kiting Grobbulus, and if you're really lucky, they'll coach the other tanks on what to do while you're handling other problems.
4. Take the number of healers you'd need for a basic, competent raid. All set? Now add another one.
Good healing -- or, in its absence, a lot of healing -- can hide some of your raid's inexperience with the encounters, and if you're pugging a 25-man run, it'll also help tremendously once you hit fights like Heigan, Grobbulus, Gothik, and Sapphiron. There comes a point at which you start to hit diminishing returns on this -- i.e. there's no point overloading your raid with healers if you won't survive Patchwerk and Thaddius -- but a full complement of 7 healers would have been a godsend at times.
5. Get Failbot and configure it to send information to you or the raid leadership.
Failbot is a tiny mod that will let you know instantly if someone's crossed charges on Thaddius, has two left feet on Heigan, or has an uncanny attraction to void zones on Kel'Thuzad (among other things). No matter how good you are as a raid leader, you still have to do your job in raids on top of trying to watch how things are going elsewhere. If you're not using Failbot, try to use and/or configure other mods to give you fast, simple information on how players and the raid are doing overall. If someone is clearly having problems on a particular fight and you know they're new to raiding, you won't lose anything by messaging them privately and asking them if they have any questions before you reach the next boss.
6. There's no perfect loot system for a PuG raid, but consistency is important.
/Roll subjects you to the tyranny of RNG and how lucky -- or not -- players are on a given night. Loot council is stupid when, by pugging, you've tacitly admitted you can't rustle up enough of your own players. Telling players to talk amongst themselves to figure who needs an upgrade most is just asking for trouble, and I'm equally uncomfortable with a third party trying to make that call. While the stars might align on any given night and one of these systems might result in a perfect set of drops going to the most deserving and grateful players -- don't count on it.
We wound up using Master Looter and a version of /roll that gave greater priority to main spec, main-toon players over alts and off-specs, and greater priority to players who hadn't previously won anything during the run. 22 out of 25 players ultimately left with upgrades, and in a lot of cases, multiple upgrades and even some offspec gear. I didn't get any serious complaints and people seemed happy with the arrangement, but it was occasionally very hard master-looting a piece to someone who I felt wasn't as deserving as a more hard-working player.
Whatever loot system you're using, announce it before the run begins, and stick with it. You may not like the individual outcome all the time, but one of the most frequent complaints we get on the site is that a PuG group or raid decided to change the loot rules on the fly. If your pugged raiders didn't like the loot rules, they didn't have to come, and there's nothing that says you have to like the individual outcome of each decision as long as you're consistently enforcing the process by which they're made. Having to master-loot a tier piece to a 700 DPS player over his healer comrade just because they have equal priority and the former won the roll will make you want to pull your hair out, but that was the system you all agreed on before you started, and as any lawyer could tell you, hard cases make bad law.
Master-looting a piece to a more "deserving" player is a great way to get the entire raid to think you're a dishonest prick (and they'd be right). Incidentally, it's also a fantastic way to end up on Guildwatch.