Getting the invites out on time goes miles toward keeping everybody happy. It's important to remember that raiding takes time -- time that players have set aside to raid every week -- and nothing heats up the blood of a raider more than having rushed home from work and plonked down in front of the computer just to see that their leader isn't online and that nobody's heard from them. Emergencies happen, and in the end WoW is just a game, but a truly awesome raid leader figures out a way to let somebody else in the raid know that something came up.
It should go without saying, but a raid leader really needs to know the mechanics of the bosses in front of them -- or at the very least has somebody else in group who can explain for them.
If loot gets mis-distributed, make sure everybody else knows it's being taken care of. If the highest roll doesn't win for whatever reason (if the high roller already won something, if a DK won a roll on a spellpower neck, if a healy priest rolled main set on hit gear, or any other reason), explain why they're not getting the loot. Things get ugly without timely explanations.
On the other side of the coin, a big part of a raid leader's job is pointing out what went right, partially so it happens reliably and partially so the raid feels like they're making progress on difficult encounters. Never underestimate the value of a pat on the back.
Knowing when to take it easy and knowing when to push
Some people like raids run by a drill sergeant; some people like a three-hour stream of fart jokes. Most people, I think, like something in between. Humor is a good thing; humor on Saurfang when the blood beasts are getting stuck in the melee, not so much. It's up to the RL to keep everybody focused when it counts.
People raid for different reasons. Some want progression, some want rep, or gold, or gear, or achievements or (frequently) some combination of those. A solid raid leader takes the needs of the entire raid into consideration and tries to keep everybody happy.
The faster a group moves through a raid, the more time can be spent actually downing bosses. Little things like getting loot distributed while pulling the next set of trash keeps raiders focused and involved while letting the momentum of a fresh boss kill do its thing. Alternately, if you've been wiping on Putracide for hours and you're not getting anywhere, your raid leader should know when to take a step back, breathe deep and change focus to something else.
It's sad, but true: Some raiders don't play well with others. There are all sorts of stereotypically bad raiders, including but not limited to: the druid who refuses to get on Vent; the shadow priest who stands in fire; the mage who routinely pulls aggro; the pally tank who forgets to throw up Righteous Fury. Some of those things can be mitigated by uber gameplay, but some (and I'm looking at you, Not-On-Vent Guy) make everybody's raid harder. Kick and replace shouldn't be the first line of defense, but a raid leader shouldn't be afraid of it, either. Raid leaders make the tough calls for the sake of the raid so the rest of us don't have to.
The ability to learn from failure
Nobody's infallible, and when things go wrong (and they frequently do), it's important for a raid leader not to get frustrated. The healer who forgot that his big heal was on cooldown and let the tank die knows what he did, and he feels bad enough about it without the RL calling him out for it in raid chat. Shame isn't encouragement; a raid leader's better off dusting everybody off and moving on, whispering the offending party if it becomes more than a one-time problem.
Really, the above guidelines are just variations on one theme: A raid leader needs to do what's best for the sake of the raid in as efficient a way as possible while remembering that the other raid members aren't NPCs. In other words, if you want nine people to follow your lead, don't piss them off.
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