It is very easy. I got bored pretty fast and stopped playing.
They game also pushes onto the player that a non-lethal playthrough is the "right" way to play. I hate when games that claim to be open try to shoehorn a player into playing a certain way.
I agree with this as well. The game gives you all these toys to be lethally very effectively, and then constantly chides you for offing bad guys.
The other dynamic I didn't like was the heart/rune/bone charm dynamic. I felt that finding the items was more important than actually paying attention to the mission, which sort of diluted the game for me.
I like the way the first Bioshock handled awarding power ups by making new tonics available after the successful completion of a mission. It makes sense to tie the power ups in a game into the story, and not just have the best rewards in the game be a tacked on gathering assignment.
Plus, having to switch back and forth between the heart to find the the charms is annoying. This was about as much fun in Dishonored as being a flashlight ninja was in Doom 3.
Overall it was a decent game, but I was definitely ready to be done with it by the time I got to the end.