Not the most original setting, but the mechanics more than make up for it.
Half-Life 2 is here as well, and lends the most obvious influence. I almost slipped and called the City Watch the Overwatch when describing the game to a friend, they're that similar. They even have "stilt-walkers" reminiscent of the Tripods from later in the game. The similarities are impossible to deny. Even the quasi-industrial architecture melded with regular buildings that was the visual theme of Half-Life 2 makes its way into this game.
Thief, however, is probably the most obvious influence on Dishonored. Stealth is the watchword here. While you could feasibly run and gun your way through the entire game (none of the enemies are particularly difficult to kill, especially in the early game) the sheer joy of sneaking past a few inept guards or knocking out a guard and stuffing his sleeping body in a dumpster is not to be missed. Going through the levels and completing them without being seen at all is far more rewarding then you would imagine it to be, and finding increasingly complex ways to use your powers to scare the hell out of NPCs is just plain fun.
Also, one thing I like about this game is that the NPCs aren't "dumb". You can easily escape from someone that's spotted you using your powers, however, they don't simply "forget" they ever saw you. A guard that's spotted you will alert other guards he passes with a quick "Head's up, there's a prowler about". The more blatant you are, the more actively the guards seem to search for you. It's a great addition to the stealth genre and reinforces the realism just that much more.
And yet, for all the "homage" that this game that this game is guilty of, a few things make this game shine. For one, the combat is stellar. Battles are quick, bloody, and believable. You actually feel like the enemies are trying to kill you and not like they're just AI controlled obstacles to be run through. They parry, they dodge your powers, they attack in concert. In fact the AI was so good initially that I was actually disappointed when the enemies didn't use the environment to hide and mount their own counter-attacks. When you get down into nitpicking, you know the system is good.
Another thing that struck me was just how open this "linear" game is. You have multiple options for completing each mission. Usually there are lethal and non-leathal options, but most missions even have multiple variations of both of those as well. It makes for a rich immersive experience in which you are in total control of your destiny. This is where the game starts to leapfrog Half-Life 2. The sheer reality of the world Corvo inhabits drew me into this game in a way Half-Life 2 never did. It's visually rich, sure, but a lot of games have great graphics. You can tell the developers of Dishonored spent a lot of time thinking about how to bring their world to life, rather than just add more bloom and HDR effects.
The voice acting in this game is far above the standard. I won't spoil it for you if you haven't looked up the cast, but some A-list Hollywood talent is here, and Granny Rags in particular was a delight.
One tip - equip The Heart and use it often. The insane amount of extra dialogue is icing on an already stellar cake.
If you're the patient, thoughtful type that doesn't fare well in "twitch" based First Person Shooters, or you're just tired of waiting for Half-Life 3, give Dishonored a look.