I too have played it a bit, the highest i ever got was level 40, so i was not a hardcore player at all. But i had 5 or 10 level 20 to 30's, so i know a fair bit i guess.
It's fun because there is so much to do.. you can just go sit down in a bar drink beer with your real life friends, and literally just talk for hours - in that aspect it's like the best Instant Messenger ever created.
Again, endless amounts to do. I have never been in an MMO with anywear near the amount of content/items/quests that WoW has. There is always a new peice of gear which looks cool that you want and you're prepared to work for it.
Beautifully designed worlds, the level design is unparalleled. Even 5 years on some of the places just look incredible, there so well designed.
There is always a sense of achievement, wether you just ranked up a level or you just killed a mob 5levels higher than you, the game is full of constant rewards and praises.
The Auction house..god i loved that place. If you find yourself as the kinda person who loves to make some money using cunning plans, the auction house is the thing for you
The Auction house is very hard to learn, and to get good at. It is truly massive and the WoW economy is changing by the hour and you HAVE to be on top of it and always know what's happening to do well. My friends didn't enjoy it so much, so while we had created our own 20man guild i was sat in the auction house for 8hours most Saturdays when i had nothing to do. When we were all around level 20, the guild had around 2500Gold with me making probably 75percent of that just from the auction house. Good days
I could go on about why WoW is so fun, but i really can't be bothered. That said, i was never actually addicted. I could happily go a week without it, and come off at any time. And i usually only found the game enjoyable for 3month periods, after that i'd need a month or two's break until i could return.