Jason over at Wife Aggro sends word about a good piece he posted on a subject I don't remember ever covering: how to switch over to a new main. It's an interesting thing to cover, and a situation that I bet happens more often than people think. But as Jason says, especially since the Burning Crusade was released, switching an alt up to your main is quite a task. In vanilla WoW, things weren't too bad-- you had to run BRD once or twice, run UBRS once, and you were pretty much set for endgame. But in Burning Crusade, there are lots and lots of keys and factions and questlines and decisions to deal with, so switching is a much tougher thing to do.
Not to mention how your guild takes it-- dealing with grinding faction rep again is one thing, but "giving up" a character that your guild has helped equip (sometimes with epic gear) causes all kinds of drama by itself. And while it's probably easier if you're switching to benefit the guild, Jason is actually doing the opposite-- he's leaving his Holy Paladin for a raiding Rogue.
Of course, this is still just a game, and you should play what you want to play. If that Warrior you've got just isn't doing it for you lately, you have every right to make that Druid you've really been enjoying your main. But you've also got to remember that this isn't a solo game-- especially in guild situations, switching from an alt to a main can have a huge effect. But if you handle it as Jason seems to be doing, by carefully considering all the consequences of such a switch, both you and your guild will be better off in the end.