It's addictive because it's designed to be. It appeals to the competitive (PVP), gamers that crave a challenge (Raids, the continual addition of something bigger and badder to challenge you further, solo gameplay), people who like to feel they have acheived something (quest rewards, titles, etc), people that like to explore new areas, people that like customising their character with gear/specs/etc and it even has an element of social interaction. There's something there for pretty much any type of gamer.
Now don't get me wrong, I'm not saying it's perfect, or the best game ever, I'm just stating my opinion on why I think so many people enjoy it and become addicted. I was an addict myself a while back and the whole PVP rank system destroyed my social life completely. I was PVPing pretty much every second that I wasn't sleeping, all the while aiming for Grand Marshal. The week after I reached Marshal my net goes down for 9 days which gave me time to think. What was the point in putting that much effort into attaining a rank and getting the gear that came with it? Shortly after the net came back up I quit the game and managed to get my normal life back. 6 months later, the expansion comes out and being curious about all the new content and wanting to see what my character will be able to do, I start up again. It's fun. There's some interesting new areas, quest, etc, I especially liked the addition of bombing runs. There's a difference now though. I don't feel the need to play anymore. I log on when I want to. If the game starts to get stale and feels like a grind, I stop until something new comes out.
I've just got back from a 6 month break (ran out of other games I wanted to play) and while there's more content, it's not really that much. So I'm levelling a new character that's completely different from my main. It's a nice new experience and a different way of playing which gives things a different feel and prevents boredom setting in. Looking forward to the expansion and definately the Death Knight. It should be fun exploring the new continent, learning a new class and seeing all the little additions, but it will eventually get to the point where it becomes a boring, stale, grindfest again. Doesn't mean I can't enjoy it up that that point and then stop playing though.
As someone said earlier, only play it if you find it fun. If it feels like work, you start to prioritsie it over your real life or you find yourself getting stressed about something trivial in game, then you're taking it far too seriously and should really ask yourself why you even bother playing.