Maybe I'm just clumsy AND stupid (or lazy), and I play on the Elite difficulty, but
a ) the enemies are tougher. And it's not just they deal a lot of damage (that's the difficulty level), but you pretty much can't approach the butchers from the front. Even when you're playing violently, you STILL have to be stealthy. I guess you could say the main game contained different enemies, like the tallboys, which were hard to kill, or watchtowers which couldn't be killed at all, but those were few, while the butchers are a common enemy in the slaughterhouse.
b ) puzzles. In the main game, whenever you came across a locked safe, the hint to the combination would be right there, next to the safe, with very few exceptions and the puzzle would be really obvious. I didn't manage to open either of the safes in the first KoD mission and I left 1 rune there and 1 bone charm (I know the rune was in the safe, but I still don't know where the last bone charm is). The removal of the heart makes hunting for bone charms and runes a bit of a challenge for once.
I found it all to be a good thing. Don't know if it's the same in the other missions, I've only played the first, I'm trying to take it slow and savour it, heh