World of Warcraft is truly an amazing game if you give it a chance at the start. You need to level a character up to at least level 20 before you can really grasp what that character is like to play, and you might end up going through 3 or 4 characters before choosing a class you really enjoy. I had the luxury of being in the betas and was able to level a few characters to level 16 - 18 before the launch so I already knew at launch that I wanted a Night Elf Rogue. When my Rogue hit around level 42 I started levelling up a Priest alongside my Rogue because my Guild needed healers. I enjoyed the Priest thoroughly even though I knew nothing about them when I first started levelling him, and now out of my two level 60's, my Priest is my main character and I use my Rogue for hunting / farming / battlegrounds. There is never a time where I log into WoW and don't have something to do. I can go and run some level 60 quests, I can farm for cash to buy nice stuff, I can raid with my Guild, surf the AH for a bargain, chat to friends, whatever I want. It is never a chore.
It is the only game that I keep going back to. I've bought loads of games for my 360, PC, PSP and DS but they hardly ever get played because WoW is just far too enjoyable. Ok, so it's not for everybody, but there is so much to do and it does have a lot of elements taken from other genres which makes it interesting.
I don't play it every night as I have a lot of other things to do too, but on the nights I do play it, I don't want to go to bed and wish I didn't have to go to work the next day because I just want to engross myself in the world of Azeroth
My advice is that if you're going to give it a try, give it a proper go before making a decision. Play a few characters in your 30-day free subscription and try to reach at least level 15 - 20 on them before you make your final decision on class or whether you love the game or not. If you do this, you'll be able to really know whether it's going to be for you or not.