An overall lack of effort in its writing and central mechanics ruins what could have been a promising game.
The central plot of Dishonored, as far as I can discern, is that you are framed for the murder of an empress by two dickheads who wish to rule over a city infested with plague and carnivorous rats that can consume an entire human body in a matter of seconds. (Incidentally, I only know they were directly involved in the plot because in true Bond villain fashion, they specifically kept me alive to tell me this, never once considering that I might escape or someone might overhear their boasting and turn them in for treason.) After escaping from prison, you get contacted by a resistance group that wishes to put the empress' daughter - kidnapped by the assassins that slew the empress - on the throne, and to do that you have to investigate the conspirators in order to foil their plans and discover her location. They also give you a skull mask to conceal your identity; you may be a criminal condemned for regicide, but I'm sure nobody's going to report some suspicious figure in a creepy mask climbing around on the rooftops. They never do, by the way.
Incidentally, the plague and deadly rats elements tie into the game's "moral" mechanic, encouraging you to utilize stealth in order to avoid and bypass enemies - dead bodies attract the rats, you see, and you're trying to prevent the city from being overrun. Only this fails to make any sense when you consider two things.
A) The villains have the technology to build energy barriers that instantly disintegrate any organic matter they touch. Why not just toss the bodies into the disintegration fields instead of letting them pile up in the streets or on barges?
B) Your character can buy a power only minutes into the game that rewards kills - first only stealth kills and then any kills - by disintegrating the bodies. Kind of undermines that central mechanic, no?
But a paper-thin premise, cliched plot and disconnect between writers and designers are the least of this game's problems. Let's talk about the stealth mechanics, which are clunky and poorly implemented. The guards are the most oblivious enemies this side of Metal Gear Solid - stabbing a guy in the throat, so long as he didn't hear you approach, causes nearby guards to write it off as "hearing things", then dismissing it seconds later as wind. The distance at which guards can see you seems to be completely sporadic - sometimes you're completely hidden at ten feet in a well lit room, and other times you're 100% visible at fifty yards in total darkness. Staying unseen is pretty much just a big game of trial-and-error until you buy another power that's available right away, which I'll detail in a moment.
Combat in the game is oversimplified and ultimately lacks any degree of strategy. 90% of enemies can be eliminated simply by putting your back to a wall, holding the block button, then mashing the attack button once their attack bounces harmlessly off your blade. The other 10% can be taken down in a single shot from a flintlock or crossbow, which you find a hefty supply of ammo for throughout the city. Rather than rewarding you for being stealthy by making your enemies a formidable threat, it's instead often far easier to just bulldoze your way through groups of enemies. Funny, I thought the central tenet of the stealth genre was that enemies could easily overwhelm and kill your character, hence the general idea of avoiding confrontations.
In a desperate attempt to make the stealth mechanic workable, Bethesda gives us another unbalanced power right out of the gate. Namely, "Dark Vision". This is essentially the detective mode from Batman: Arkham Asylum, allowing you to sight enemies through obstacles from a considerable distance. The problem is that in Batman, it provided a handy way to keep tabs on rooms full of gun-toting enemies (who could still eliminate you in no time flat if you were sighted). In Dishonored, enemies rarely carry guns, and thanks to their aforementioned oblivious nature, it becomes child's play to simply sneak up and stealth takedown / convert them to ash one by one. So rather than being a well-implemented mechanic that gives you a chance to spot rare opportunities to strike, it just radically unbalances the game in your favor.
It also made sense that Batman would have have such an ability, technological wizard and badass super-ninja he is. Dishonored's version, on the other hand, is a deus ex machina power given to you by some cliched mystery man. I'm sure his presence is explained later on; the problem is that the game does nothing to make me care because, as mentioned, the story in Dishonored is riddled with contrived conveniences and bland, cliched scenarios. In fact, I bet you that I can guess this character's true role right now: He's the main villain and he's trying to get you to sow chaos in the city with these powers because he's EVILLLLL!
Update: Nope, he's just a boring deus ex machina with no bigger role to play. How droll.
It should also be noted that, in spite of the numerous options for quick, clean kills and takedowns, you're also given little "events" to assassinate your high profile targets. For instance, one gives you the option to swap a poisoned glass of wine (intended for your target's victim) with the target's own glass. You can also elect to poison both of them, or even neither at all! Somehow the first option is the least "chaotic" of the three, because apparently a poisoning of a high profile city official won't attract as much attention as, say, shooting him in the face with a flintlock. Also, the event plays out exactly the same regardless of how much of a ruckus you made to this point - noisily shooting and stabbing guards one room over will in no way tip them off to your presence.
Finally, for a game that asserts a freedom-based mission structure a la Thief, there is an awful lot of hand-holding going on here. Your goal and all optional mission objectives are clearly pointed out via convenient beacons. Hell, even hidden items - usually a reward for diligent exploration of an area - are pointed out for you by, you guessed it, another item you get very early on in the game. There's not really even a chance to overlook them, because whenever you get within 100 meters or so, the game will actually tell you to equip the heart in order to discern the exact location of the hidden goods. So what could have been a good chance to introduce some replay value and encourage exploration is rendered completely moot. Way to go, Bethesda.
In short, Dishonored is a bust. It tries to implement elements of Deus Ex and Thief, but fails to match any of their gameplay merits, their interesting storylines, or their sense of rewarding exploration and replay value. Pick up any of those games instead and skip this turkey.