Addiction

Why is Warcraft so addictive? Let me count the ways:

1) The game engine itself is complex and well designed.
At level 1 of course the game is simple, but by the time you reach level 80 the optimum rotation is usually challenging. PvP, for all its current imbalance, requires and rewards skill, while PvE requires tactical thought, co-ordination and (to a lesser extent) everyone to play their class well. It's a fun game to play.

2) The game is beautiful
There are very few games that approach the status of art. The GTA series of games are certainly art, and warcraft is certainly not, but at times it does come close (from moonrise over Loch Modan, to the Northern Lights in Icecrown, with many highlights in between).

3) The game is social
I would have quit the game long ago if it hadn't been for my guild (who I've been with for over 3 years now). I've met some interesting and friendly people, and I've known them (virtually at least) for a long time.

4) Respect
How easy is it, in the real world, to get respect from people? It's quite hard I think. You have to impress them with your force of personality, your physical ability, your charm or your intelligence. You then have to make them aspire to emulate you. In Warcraft all you have to do is hang around in Dalaran in your T7, while riding on your Amani Warbear.
I remember a UBRS pug a couple of years ago. We were having the usual problems dragging a bunch of no-hopers through the dungeon, and our healer left. I asked my druid friend (now a tank, but healer spec at the time) to join us. He turned up in his full epics with 4 or 5 pieces of T1 gear, basically looking like something that had been dragged through a hedge backwards if I recall how the items looked. Our group leader immediately said "Now THIS is a healer" and began deferring to him in every decision. I laughed a lot about it, but by coming along to a few 40 man raids (and in those days you would drag anyone along just to make up the numbers) he immediately earned the respect of people who hadn't done that content.
Nowadays I have 25k health unbuffed and epic/blue gear. This has been mainly earned through grinding heroics but even this mediocre gear is sufficient to get me preferential status and respect in pugs.

5) Goals
In real life, if you do something positive, or that requires hard work, you don't often get an immediate tangible reward. In warcraft you often do - gold, rep, purplez, whatever. It feels like you've achieved something if you gain 5,000 honour in a day, whereas effectively you've been playing tetris for three hours and have achieved precisely nothing. The illusion of actual achievement is very powerful though.