The Secret World is not your standard MMO. It's not just the setting, but the questing, the leveling as well as the entire atmosphere is vastly different. I've only played through the first 2 beta weekends, but so far I'm really liking how different it is from other MMOs.
First the questing, unlike most MMOs many of the quests require you to actually think, navigating a booby-trapped dungeon, figuring out a computer password using clues and google, even killing your character off to talk to special NPCs. It just plays very differently to other MMOs.
Second is the leveling, you don't have traditional levels, only leveling up of skills. Considering that you can only have 7 active and 7 passive skills, you end up not focusing on leveling (unlike other games, it doens't feel like a means to an end) so I play because I enjoy it, leveling up just feels like more of a bonus.
Third is the atmosphere, the monsters and settings actually look pretty creepy (aside from zombies, which a re a bit boring). Deserted building, following blood trails, flocks of ravens etc. Really works well with the modern setting and sets a nice eerie mood that's pretty unique to this game.
Last thing is the community, unlike other MMOs, instead of outright telling you every answer, people generally give you clues to give you a chance to solve the problems yourself rather than outright answers, and I really love that about the community
With the emphasis on problem solving. It might not be everyone's cup of tea, but it's truly the most unique MMO I have seen. Playing it I often actually feel like a secret agent or a spy, and the sense of achievement when you solve a problem yourself is pretty awesome. Basically, if you are interested in trying something different, and problem solving is your thing, then it's worth giving this game a go.