It's fun and addictive, but there are six main things you should be warned of.
-In most RPGs, you can chose to be any class but still use some different powers. In WoW, if you chose to be a warrior, you'll kick ass in physical combat but you won't be able to use any magic at all. Likewise, if you play as a mage you'll always end up as a support class.
-The Horde/Alliance fiasco is interesting, but it's also quite annoying at times. People on the other team will often spawn kill you, camp your corpse, and just be idiots.
-Leveling up, making money, and overall developing your character takes a long ass time. Nuff said. I've been playing as my rogue for two weeks, and I only have two gold, which is not a lot.
-It's the same hack and slash repeated again and again. All the quests are (bassicly) the same. Either you're sent out to kill someone, you're sent out to gather items, or you're sent out to investigate.
-The players on WoW are idiots. Since most of them are hardcore MMORPG gamers, they have their whole "Holier-Than-Thou" attitude. If you ask for help, you'll get brushed off. If you get into a stupid debate by them, you'll ineviatbly lose by default. They almost always talk down to you, unless you have experience with FFXI or Asheron's Call or something.
-Loads of tech issues. It took me about three days when I first installed WoW to get everything right, and to this day I still have to ocasionally re-upload. Having to download patches every Tuesday is a pain, too.
besides that, it's an amazing game. ^^;