I agree that it's too short, and the main characters were a tad bland (I thought Emily was alright though. Also, I like how she changes depending on your actions), but I actually thought the guards were pretty well characterized, with solid dialogue throughout the game. They're pretty human, with different views on politics, differing amounts of empathy for plague victims, and even different opinions of Corvo depending on whether he's been massacring them or mostly letting them live. In any case, I don't think the blandness of the characters matters too much to the game. None of them were really important enough that their bland personalities harmed the game, which is quite excellent at its core mechanics.
I didn't mind the binary choice aspect, but the urge to reload the game when caught instead of outsmarting the guards did get pretty annoying. The devs ought to patch out the "getting caught increases chaos" thing, and also add more stuns (plenty of ways to murder guards, only a few to knock them out, which is strange considering the game seems to want the player to knock them out).