I've been playing on normal and got to a battle that I just couldn't beat (I died probably 100 times in this one part!), so scaled down to easy for that scene. The main difference I noticed was that it aims a bit for you. It's very subtle, but when I pull out my bow and press on the trigger to find the "red" target, it's much more likely to already be lined up for me while on easy mode. The bad guys can still move out of the way, but I don't have to waste time lining it up in the first place (something I'm not terribly good at).
In the non-combat zones, I haven't noticed any difference at all. I don't think the playthrough is different on different levels - just the way the guns / controls handle seems to be different.