I really liked the world they built for this game, the art design is very unique and gives it a lot of personality, especially its beautiful and atmospheric exteriors, and the backstory is quite original, giving you an extra motivation to explore the levels just to have a look around.
As a stealth game, which is how I played it, it was a fun experience. The game mechanics are simple but work well, and the powers you can get give you enough options to keep things interesting. My only minor complaint is that I wish there was a way to check the current mission's stats to see if you've been detected or killed someone up to that point (there are visual and audio cues when those things happen, but they are sometimes easy to miss). I couldn't get the no kills achievement because after one of the missions the game said I had killed one hostile, and not knowing how or when that had happened the only solution would have been to start the mission again from the beginning... A small detail, but it somehow annoyed me more than it should.
More info: https://en.wikipedia.org/wiki/Dishonored