That's the thing that I've noticed about the game - if you go right off the bad for the objectives (I've done that on Mission 1, before deciding to replay it), it goes on really fast and it wasn't really enjoyable. However, when you immerse yourself into the game and start testing out boundaries and limits of the game, when you experiment with different abilities and tactics, that's where the most fun lies... Thus me spending so much on a "short" game. This game has a lot of "secrets." Okay, not exactly secrets, but a lot of hidden really cool stuff (and insights) which make it much more enjoyable as if you'd just blast through the game with doing most of the objectives, the game would seem average at best as storyline, while definitely makes sense and seems like it's not forced, is very predictable.
I'm actually surprised I enjoyed it so much as I generally play games where game's plot is the main focus (Because if game has a shit plot, I normally discard it really fast), but Dishonored managed add a lot of interesting additions to the sideplots and made the gameplay really, really enjoyable, which kind of offset the bigger negatives.