I really wouldn’t worry much about starting stats. Once you get out of the beginning levels, it really doesn’t matter that much. Racial bonuses matter more.
Whenever we put together a group, we seem to have the hardest time finding a good tank (generally a warrior or a druid, sometimes a paladin) or a good healer (generally a priest, paladin, or druid). Every server’s different, of course, but we seem to have no lack of mages, warlocks, paladins, and rogues. And I never hear groups saying, “gee, we need a hunter or a shaman.”
If you are a good tank or healer, you will be in great demand.
Yes, an idiot can cause a wipe. The way WoW tends to work, though, is that you connect with a guild and learn who knows how to play their class and who doesn’t. When you form a PUG (pick-up group) for a lower-level dungeon or quest, remember who the good ones are, and talk to them later for the tougher runs.
When you get an idiot on a run, dump him. We were doing a run the other day with three priests and a warrior. The holy priest healed, the shadow priest (me) did damage, and the third priest pinch-hit whereever she was needed. The warrior simply refused to wait for us to regain mana between battles. I tried to explain that “healer out of mana = dead warrior” and he still didn’t get it. Next time he charged into battle when we weren’t ready, all three of us left the group and hearthed out, leaving him in there alone.
It’s all a matter of finding the right spot. Buy large bags and good gear and plan ahead: go out with your bags almost empty, plenty of potions, good scrolls to buff you up, and get quests in the same area where you’re going to grind. The great thing in WoW is that any class can grind solo.
You will probably end up respeccing at least once. For example, shadow priests solo better and level faster than holy priests, but holy priests are in more demand in higher level dungeons.
Sometimes, respeccing makes it feel like you have a whole new 'toon, with different play style and strategy.
A final tip: you may be encouraged to get a mod called “damage meters.” Do not use it as anything other than a rough guide to performance, and especially don’t trust mages that talk of nothing but how they did on the meters. A warrior doing his job doesn’t have to cause much damage–he just has to hold the aggro. A mage should be doing crowd control (sheeping, freezing, slowing, counterspelling…), which doesn’t cause damage. Ditto with a hunter setting his traps.