WoW sucked up a ton of my time the last couple of years. I know enough about the game, I think, to give you a basic idea of why some people claim it's so awesome, and others (including me) think it's not that great.
Here are 4 points that WoW has going for it. Then, in italics, purely opinion on my part.
1. First of all, the game is dang easy to learn. The first ten levels of the game are nice and slow, in a protected environment, giving you plenty of time to learn your character. It does a great job of giving you a purpose, a little story, and just enough reward to hook you on the "I need more phat lewtz" idea. In other words "instant gratification".
This is the biggest reason that WoW got so popular. Anyone can sit down and play it. It was a nice change for the genre being able to start having fun without needing to "work for it". Many older MMO nerds will tell you, however, that having to work for something makes your accomplishment that much sweeter. This will be a unending battle between WoW fanbois and the rest of us. That said, this one huge attribute of WoW made Blizzard rich(er)
2. You can get something accomplished in a small amount of time. The trip to level cap is great fun. You can always log in, do something meaningful, and be done in 45 minutes if you like. Caters to the casual gamer.
This works great until the endgame. see point 4
3. No harsh death penalties You lose nothing if you die. No XP debt, no money, no items. Only a little time and a blow to your pride. In this way you are always making progress. It's a very friendly game to everyone, at least in theory. I point to the battlegrounds as a reference. Absolutely no way to lose anything by joining a battleground. If you lose, you still make progress.
People who have only played WoW will tell you how much of a turnoff death penalties would be to them, if they were to think about another game. I think this is unfortunate. Having something to loose when seeking to gain makes nearly everything that you do a lot more interesting. I'm not going to go on about it (you can find many reasons why many people like death penalties from any whiner in a developing game's forums) but I want to point out that people can just as easily ruin your fun. But in WoW, there's hardly anything you can do about it. blah blah death penalties actually reduce ganking blah blah.
4. Raiding Endgame Getting together with 40 people in the game and taking down bosses for items is the tried and true MMORPG way of keeping people subscribed. Frankly, it can be fun.
You almost HAVE to raid once you hit 60 in WoW. First of all, it's nearly all of the content that gets added. Second, WoW is so item-centric that you can't even have fun in the battlegrounds against people who raid. This is a complete turnaround from the 1-60 trip. You join a big guild, do the same raids week after week, each requiring a substancial time commitment. We're talking 4-7 hours here. It's not the same game.
Because of this, people end up rerolling (starting a new character) and/or not really even talking to people outside their guild. It's wierd. The same "hardcore" people that complain that WoW sucks, are the same people that the game caters to at the end. It's addicting though, I admit to that.