I consider Eternity an improvement on most aspects when it comes to game mechanics, and it's hard to isolate specific mechanics that I enjoy more than others, but here are three things that come to mind right now:
1) The resting mechanic. I never really rest spammed in IE games either, but the way Eternity handles things has still improved my gaming experience a lot. It adds excitement, feeling of attrition and urgency, and strategic depth.
2) The health/endurance mechanic. I love it, I think it really succeeds in combining the best aspects of traditional and regenerating health, and it has some interesting features of its own. I like how well it handless long-term attrition and how it doesn't just eliminate perma-death entirely, even though you don't have to worry about it most of the time. I like how it puts a limit to healing and even makes it a double-edged sword: with enough healing a character now risks death instead of just getting knocked out. Just think about that from in-universe perspective: even with magical healing there's only so much your body can take, and sooner or later your body will simply fail and there's nothing that can be done at that point.
3) The four defenses. This one's a bit of a dark horse and probably not utilized in-game quite as well as it should be, but I love how status effects lower specific defenses making the enemies more vulnerable to attacks targeting those defenses. This is a huge improvement over the way saving throws worked in IE games. I don't think this system really reaches its full potential, but perhaps that's because I just haven't been able to pay attention and take advantage of that system the way I should have - probably because there are so many options that it can be a bit overwhelming. I suspect that the majority of players mostly just ignore this stuff, but there's some huge tactical potential there.
I could list many other mechanics, but I guess that'll do for now.