I've been playing it since 2006, and I personally think its the best in its genre. Meh, here's a checklist.
- It truly is a World of Warcraft. The whole environment is really massive and all the zones are varied enough that you never run out of nice scenery. The fast travel system (Gryphons/Wind Riders) involves you flying over and seeing all the zones and players on the way. This sounds simple but there is a lot to be said about immersion.
- The combat is pretty good. Gone are the days of standing there repeatedly hitting the same thing with one key. There are a lot of different spells and abilities your character can use, and all the classes are fun. Even the "underpowered" ones.
- There is a lot to do in this game. You never have a situation where you are forced to just sit in one spot and kill the same thing over and over. There are dungeons you can run, there are always quests you can do. Hell, you can even go pick fights with random people, whether it's dueling people on your side or fighting the opposing faction to the death.
- Immersion. I touched on this a little with the environments but the questing system is really good. There are always quests you can do, and a lot of them have really interesting stories.
- It's Blizzard. Let's face it, Blizz doesn't care about taking themselves too seriously. They are perfectly willing to put in all kinds of crazy jokes throughout the whole game. There are innkeepers named Floyd Pinkus and random citizens like Haris Pilton. Players can walk around with "Jenkins" after their name. There's a belt you can get called "The Tenacious Defender." This kind of humor really adds to the atmosphere of Azeroth.
- Accessibility. This is the big one. While MMOs before it had a high commitment to enter, WoW invited everyone in. The system is simple enough that everyone can enjoy it. There is barely a penalty for death and everything in the game is really inviting. There is never a point in the game where they give you nothing to do, and there is a plethora of content for everyone. Blizzard has also sped up the time it takes to level, so getting to 80 takes about the same amount of time that getting to 60 took back in the day.
- Depth. Easy to learn, difficult to master. WoW can be as simple as "Spam 1 and 2 until the thing dies." Or if you decide that you want to get into the whole theory-crafting, there is a lot of thought and math that goes into gear and talent specializations.
- The whole social aspect. Running dungeons with your friends never really gets old, and if you don't think you will make friends in this game then you are mistaken. I'm in a guild right now where are entire raids consist of a bunch of cool guys talking shit and hanging out. It especially helps if you have real life friends playing with you.
There are a lot of people that talk bad about WoW, but most of those people either had a bad experience, or have never played it. Yeah, some people would call it a second job, but it only becomes a second job if you want it to be. Yes, you may very well lose your life to this game, but believe me, you won't see it like that. You'll be too busy having a good time.