I originally killed a little too much but on a certain bridge mission, I killed only a couple thugs.. strangely satisfying indeed to let most guards/thugs go. So many reloads hehe.. but was not frustrating at all .. incredible game!
On my first playthrough I played brutally, just hacking and slashing at everything in sight, and exacting maximum revenge. Second playthrough I did ghost/clean hands and found it was actually more enjoyable. It really tests your skills since you cannot be seen at all. You have to think carefully, analyze patrol routes and find creative ways around them. Having said that, playing through the first time was ridiculously fun as well.
Sorry it took a while to reply. Yes, I may try to do a serious stealth attempt sometime in the future. I heard the effort adds double the hours needed to complete it, which is a great way to get your money's worth.
Yeah, that's the only downfall, because the weapons and powers are extremely fun to play with. But you can always blast some dude's face off and then reload. I did that so often when I did my stealth playthrough.