In Dishonored you play Corvo, the ex-bodyguard of the Empress in the city of Dunwall. Wrongly accused of her murder, you are forced into hiding and become an assassin driven to expose your accusers. Corvo has some help with this as he's been given supernatural powers by a mystery force called 'The Outsider'.
Dishonored's Dunwall is a mixture of British Victorian and steampunk styles, so you have classic architecture with Jules Verne-like retro-futuristic machinery. The design in the game is fantastic, and really give Dishonored an interesting atmosphere - Dunwall is ruled cruelly, with the poor segregated and policed by violent steampunk man-machines.
Playing Dishonored takes some getting used to. Corvo's powers complicate the controls, but are great fun once you get used to them so are worth the trouble! You can stop time, possess people or animals, see through walls and teleport short distances. Add traditional weapons to this, and you have a fantastic arsenal at your command that you can use really creatively.
The level design is both ingenious and beautuful, especially during the earlier levels. While it's not a fully open world, each level is big, and there are a ton of things to see and places to explore in every one. Finding stealthy routes into a target is really fun, and can get extremely tense as you wait for the right moment to silently slip behind a guard, undiscovered.
Like Deus Ex: Human Revolution, you are encouraged to play the way you want. You can be as stealthy as you want to be, and as violent. The more violent you are, the more chaos you create, which means more rats, more guards and an even darker atmosphere. It may even be possible to beat Dishonored without killing anyone, if you can use your powers intelligently enough.
Dishonored looks great, but its stylised graphics may not be to everyone's tastes. The people have exaggerated features, making villains look more villainous and so on. There's a lot of blood, but it comes in cartoonish spurts. The city of Dunwall is the real star though, full of details that enrich the story. It's a complicated complex of streets, ledges, tunnels, buildings and more. Unlike typical first person shooters, Dunwall is an environment you won't rush through, but rather you'll get to know it intimately.
The length of Dishonored really depends how you play it. You could zip through, but that would miss the point. Enjoying Dishonored comes from taking your time, and using all the tools at your disposal.
With all the different ways you can play Dishonored and the multiple endings, Dishonored is a game you'll be able to play through more than once. It's great to have a brand new title to play in a season filled with sequels.