My realm's seen a number of guild instability issues of late, which is something most of us have come to expect with an upcoming expansion. I've found reason to mull over how the virtual world differs from the real world with respect to friendship, backstabbing, greed, betrayal, honor, and how people choose to handle their problems. In my considered opinion it doesn't differ at all, and your experience ingame is largely determined by the network of players assembled around you, whether that alliance is a recognized one in the form of a guild or simply a more informal group of friends.
So, from my own experience and with a hat tip to Pjammer, these are the people you want in your posse for the best possible experience in the game:
1. The Friendly and Easily Bored Tank.
"Man, I'm tired of doing dailies. You wanna run something?"
Wow's official forums are typically awash with players complaining about the constant lack of tanks. While we've gone over the multiple reasons why it's so tough to get a tank for 5-mans (luckily the situation is likely to change for the better in Wrath), the fact remains that there's no faster way to get a group together than to have a tank buddy around who's willing to chair your runs.
There was a lengthy period over spring and summer 2007 where I did very little ingame apart from PuG-tanking, and it dawned on me at one point that one of the biggest reasons tanks are so frequently out of general circulation for 5-man runs is they get accustomed to running with a specific group of people and, to a not-inconsiderable degree, are often "pre-scheduled" with them. As much as you may enjoy pugging (and I still do), it's tough to make yourself widely available for others when so much of your time has already been promised.
Moral? Find a tank who's gearing up, demonstrate that you're a capable player who won't make their life hell in an instance, and give them a reason to put you on their friends' list.
2. The Hunter Who Doesn't Suck.
"I'll trap over on this side. You'll probably want the melee mob Misdirected because it's not a big deal if the tank loses aggro on that caster. Healer, you'll want to stay on the side away from the trap."
I once wrote in a comment here that the first person you'd want added to a 5-man group after your tank/healer was a good hunter, and the last person you'd want added to a 5-man group was a bad one. A good Hunter, regardless of spec, excels at threat redirection, various forms of crowd control, and just plain keeping the mobs away from squishy people in the event of an emergency while providing massive quantities of sustained, ranged DPS. Outside of a 5-man, a decently-geared Hunter is a superlative farmer-buddy for people who can't farm quickly or efficiently on their own, and an equally good battleground-buddy when the time comes to farm up some honor.
It's very easy to find a bad Hunter. It's not very easy to find a good one. When you do, hang onto them, and if you are a Hunter, try to be this person.
3. The Multi-Tasking Healer
"Triangle is stunned and Wisdom is on the skull -- I'll Consecrate under you so the mob that X is going to spawn won't go anywhere. Just taunt off me when that happens and burn them down."
Paladins, Druids, Priests, and Shaman are more than just healers, even if that's what they're specced and geared to do. They have totems, various forms of bubbles, buffs, and damage reduction, Cyclone, stuns, fears, and elementals. The healing classes have more means of affecting a party's survivability and efficiency than anything else in the game. The problem with many players who are either forced into a healing role or just don't take to the job naturally is that they forget their toon never stopped being a hybrid class, and abdicate all responsibility not directly related to pressing their heal buttons. A healer who is not prepared to do anything beyond heal is someone who can probably get you through a 5-man assuming a minimum level of skill on the part of the group, but not someone who's likely to save you from a potential wipe or work with any particular distinction in a raid.
It takes time, trial, and error even for good healers to learn when they can and can't supplement their healing with hybrid abilities (e.g. even the best one is unlikely to do more than chain-cast heals if the tank is badly undergeared), so finding one with a good sense of their class, an experienced guess at what the healing load is going to be, and a little imagination is like hitting a gold vein.