This review contains minor spoilers for Dishonored.
You will do many things in Dishonored that you have done in other games - you'll sneak, you'll snipe, you'll knife guys in the neck, you'll upgrade your gadgets and throw some grenades, and you'll gawp at how elegantly you do it. What you won't do is ever forget that you're in a city fueled by whale oil and threatened by street gangs, where the rich hide behind the City Watch and the poor weep blood in the streets. You'll never feel like a visitor in Dishonored, like some impartial observer just there to cause some trouble and go. The details of life on the Isles seep unobtrusively into your pores until you cannot help but feel like a citizen of Dunwall yourself: a very pissed off citizen.
You play as Corvo, former bodyguard to the Empress and unjustly accused of her death. Shortly before you're due to be executed, Loyalist forces help you escape and set you on the path to restore the Empress' daughter, Emily, to the throne. The Loyalist forces will give you a variety of targets - some political, some personal - that need removal. Simply getting to the object of your attack will be difficult enough, as the City Watch and Overseers are no bozos, and you're a very wanted man. Your experience has made you handy with all manner of weaponry, including pistols, crossbows, and razor traps, but a visit from a curious figure known as The Outsider has given you access to magical abilities as well. Each ability costs a certain number of collectible Runes to activate, but before long you'll be facing missions armed not just with guns, but with the power to see through walls, teleport across great distances, summon wind gusts or possess animals.
Dishonored takes a two-fisted approach to confrontations, putting a sword in your right hand and leaving your left hand free for another weapon or magic. Combat doesn't require technique much more advanced than aiming and shooting, but don't think that means combat is easy. Your enemies are smart, plentiful, well armed and want nothing more than to take you down. Your arsenal never gets very large - a pistol, a crossbow, some grenades - and while you can buy upgrades like faster reloads or better range, trying to succeed by just running and gunning will almost assuredly spell your demise.
Giving you options of how to resolve your mission isn't anything particularly new, but Dishonored does it in a pleasingly organic way. Nothing feels like The Stealth Option or The Combat Option, and you're given plenty of freedom to experiment and change tactics on the fly. Possess a rat, scuttle through the vent, then pop out and shoot a guard before he can sound the alarm. Teleport behind an Overseer and slit his throat. Climb over the rooftop and use a gust of wind to blow a lowlife thug over the balcony. Or, do the really unexpected thing and don't kill your target at all - nonlethal options are available for just about every mission you receive, supplying the chance to exercise mercy, if you're in a forgiving mood.
Weigh your decision carefully, because the more victims you kill, the more Chaos you create in the city. Each level can be completed with either a High or Low Chaos level; High Chaos can lead to more guards, more plague victims, more rats, and a different ending. You won't know how much Chaos you're causing until you've completed a level, but generally speaking, leaving lots of corpses lying around or calling a lot of attention to yourself tends to get people riled up. High and Low don't necessarily equate to "wrong" and "right," but the tone of the city is certainly darker when the Chaos is High.