What I like about WoW is that Blizzard keeps adding more content, and not just in expansions, but in patches they release throughout the year. The game has evolved as the years have gone by, and in many ways it's much easier to play now then it was when it first came out. This is by design, so that players can level up characters easier and faster, and get to the end game content.
WoW can be played by many different types of gamers - from hardcore to casual and there are a lot of different things you can do. There's raiding for people who like organized questing, player vs. player (PVP) activities, solo questing and now pet battling.
If you've considered playing, now's a good time to check it out. You can play the game for free up to level 20. If you'd like, shoot me an email and I'll invite you - there are perks in it for both of us :)