WoW is an absolutely amazing experience the first time you play through to level 60 and I can recommend it to anyone that has any interest in RPGs. The world is huge and very well realized, and grinding is kept to a bare minimum. The interface is tight and logical, and you're constantly discovering new areas, skills and events. Any race/class combination is quite fun to play. However, once you finally hit 60 (~16 days actual playtime, less if you take lots of breaks and get a 'rested experience bonus') you generally get funneled into guild raiding or pvping -- both of which are being vastly improved (imho) in the expansion.
Alternatively, you can level an alt -- either to 60, or bring multiple alts to level 19, 29 or 39 and take part in battlegrounds and stuff. It's quick to get characters to 19 and you can have fun with new classes in one of the PvP instances, as you get access to a nice sample of class abilities by that time. Playing new classes at level 19 in the battleground is actually a good way to learn the class, as you tend to play it safe in PvE and rarely have to deal with challenging circumstances.
The best things about WoW are: it's a very well designed game, incredibly fun the first time through and quite affordable once you realize how much you can really play for $15/mo.
The worst things about WoW are: it might prevent you from playing any other games. My gaming time was almost entirely dedicated to WoW for about a year, and man, I'm still struggling to catch up on all the great games that I missed. Also, as with many online gaming communities, you have a heap of idiots -- the official forums: never read them. You also risk the chance of becoming addicted, which is bad for very many reasons.
You also just might not like WoW, but you'll know if you want to keep playing by level 20 and you hit 20 pretty quickly. In any case, I'd recommend waiting for the expansion pack, which will likely be out later this year, in the midst of the new console mass orgy apocalypse.
