Can't really put a finger on what the game lacks, but it felt flat for me.
It's massively flat compared to the first one. Firstly, in D1 you start off as a lowly fucked-over mortal escaping from prison (mysteriously broken out) and have no magic powers. Then you find yourself in a conspiracy to overthrow these evil fuckers and save the true heir against the odds. There's an obvious hook.
In D2 you've already been there and done it, obtained near godlike status via the Outsider, but the new evil wench drains you of all your powers in the opening sequence! How inconvenient. Guess we'll need to find hundreds of runes again then, as for some reason the Outsider doesn't give us our full powers back - just the ability to rebuild them.
Basically it's all obviously there to enable more Dishonored assault courses, not to make a serious attempt at advancing the saga. It's ok if you look at it in that context IMO.
Oh and it also doesn't help that they changed the Outsider... now he comes across as an emo band frontman. Why did they do this?