"Jesus, just look at that guy's gear! Why didn't that stuff ever drop for us, huh?"
If you raid at all, then odds are pretty good that you have a direct counterpart in a competing guild close to your level of progression. Theirs is the toon you'll want as an immediate reference for the purpose of comparing spec, gear, enchants, and gem decisions. If they seem to know what they're doing and some of their choices seem radically different from yours, it never hurts to ask why. They may have changed their gear for progression purposes, or they might have found something useful in a different spec -- or, appearances to the contrary, one of the two of you might have gotten something drastically wrong.
Either way you want to maintain a good relationship with The Rival. He will almost certainly be under consideration if your GM gets hungry for more players of your spec and class, and their guild is where you're most likely to end up if yours goes belly-up. You (or he) may not wind up the same spec when all's said and done, but if your GM asks if that person's any good, you want to know enough about him to give an informed perspective. On the flip side, you want The Rival to know what your capabilities and experience are, because I 100% guarantee that he's going to be asked about you in the event that you apply to his guild.
No matter what happens, you won't hurt yourself (or him) by keeping an eye on this guy and trying to be just a little bit better than he is at the job both of you do.
8. The Colleague
"If you want my honest opinion, the class leader's dead wrong about you need to be doing on that fight. Here's what worked for me."
A more genteel version of The Rival (unless your guild is really cutthroat or an ingame subsidary of the Corleone family), The Colleague is also someone who does the same job you do in raids. They are usually -- but not always -- an intra-guild player (those who aren't may pull double-duty as a Rival), and often -- but not always -- your spec and class. Now, they tend to be the most typically helpful whenever you're new to a fight that they've seen before and they can give you the benefit of their own experience, but if something clearly isn't working for you and you're not sure what's causing it, then The Colleague should be your first stop while trying to fix the problem.
Protect this person (or people). They're the most likely to back you up in the event that you have to argue in favor of changing the raid's approach to the fight, and if they're also a member of your guild, you're likely to be waiting on some of the same drops they are. If you look out for them, they'll look out for you.The tanks, the melee DPS, the ranged DPS, and the healers all comprise their own very distinct sub-groups in most raids, and they fail to support each other at the raid's collective peril.
9. The News and Rumor Junkie
"Don't buy that pattern! A blue post said it's going to be trainable in the next patch and that's why the price on the AH seems so low all of a sudden. Just give it a few weeks."
Some of us read the official forums purely for the entertainment value. Others do it because apparently they are professional trolls. Still others just hang around the forums and other WoW sites like Blue Tracker like it's their job (and for some of us, it is a job). If you don't have much time to devote to staying current with the numerous tweaks, large or small, the developers are planning, you'll benefit from hanging around someone who does. The game changes pretty frequently, but most of the time it doesn't do so without those changes being made public well in advance. Commodities on the AH rise and fall as a result of tweaks made to professions. Classes become more and less popular in both PvP and PvE as a result of the devs' experimentation with arena and new raid content.
If you want to be well-placed to benefit from upcoming changes rather than being at their mercy once they go live, the News and Rumor Junkie can help save you time and gold. And yes, this is a version of the asymmetric information Pjammer refers to concerning lawyers, doctors, financial advisers, and mechanics; these people all save (or make) money through their possession of knowledge that most people don't have. The nice thing about WoW is that such knowledge is usually free and out there for the taking. Like here, for instance!
10. The Techie
"Type in /menuhere and that should pull up the configuration menu for the mod. Got it? OK, what you want to do is scroll down and start clicking off the options to auto-publish to /raid and /group..."
Past a certain point, nearly everyone dabbles in UI enhancement or finds themselves with an unpleasant technical problem related to the game. Certain people have a more intuitive understanding of what's going on behind the electronic hissy fits being thrown, either because they've had the same problem or they just know how things work. Make friends with a Techie. At some point, unless you are possibly #4 and just as ridiculously lucky out of game as in, you are going to require his services. This person may be all that stands between you and a boring slew of nights trying to figure out how to get a mod or the game itself to work.
Now, what I've written here is necessarily colored by my own experiences; your mileage (as they say) may vary. But if any of these people regularly participate in your ingame life, I'd say you can count your blessings. I know I've left some people out, however. Thoughts on an addendum to this list?