OK I may have over exaggerated but I stand by my point, I ended up playing that game for 3 years and by the end I wasn't even playing because it was fun, by then it was just habbit.
Personaly I think it's the grind that does it, you do the same thing over and over too many times and you don't question doing it again.
I certainly enjoyed WoW at the time, raided through TBC and had a whale of a time in SWP, but by that time raiding was all I played for and when Wrath hit and raiding went out of the window, and Naxx etc was such a let down (for me at least) my reasons to play lessened and I realised that I was logging on, grinding some rep or something and having no fun doing it what so ever. Hence why I now dislike the game, it sucked all the fun out of itself.
But anywho, contructive info for the OP:
First of all you are better of just playing a 10 day trial and seeing what you think of the game, even better find a friend who will let you play on their account for a while to get a real feel for the game, at a low level you don't get much of an oppertunity to see what the game can offer.
If you enjoy it, buy WoW, and leave the expansions, as said above if at lv 58 you enjoy it, get TBC and level to 70 and again if you are still up for more get Wrath, buying them all at once may seem like a good idea but if you get to lvl 40 and realise you can't be bothered with it and stop playing, you have shelled out for 2 games for no reason.
Alliance/Horde - It'spersonal choice, I played Horde because I wanted to play a Paladin and the Alliance races, especialy human (casting animation...eugh) just don't work for me.
If you have friends in WoW, play with them, it will make it infinitly more enjoyable while leveing.
Leveling - it can be dull and if like me you realy just wanted to get it over and done with, there are dozens of free leveling guids on the net or addons that can make it a lot smoother and remove the "WTF Where do I go?!" aspect.
Class - for the love of god level a DPS class, I leveled 1-80 on my paladin as a healer and only when I leveled a DPS class did I realise how stupid that was.
When it comes to choosing a class that's where things get interesting, my advice? level each one to lv 10 and see which ones you have more fun with, you won't get much of an idea about what they are capable of at 80 but you'll come to understand what they do and how they do it.
Enjoy.