Haven't had much time to play so far, but here's what I can answer:
-Player speed seems to matter a bit. Since the tuner update, everything seems a bit slower, but if you create a bit of space on a dman, you can blow by.
-The dmen aren't ridiculously fast. The AI is improved a bit this year (From what I've seen), and they seem to space themselves out better.
-The whiff is definitely still there. I haven't tried many one timers but it definitely happens when my dman gets the puck off of a draw and tries to fire it around the boards.
-Breakout seems great if you know how to break out.
-Slot coverage is much better than 16. That human glitch goal isn't really there anymore, though if you leave the CPU open in front of your own net, they'll crush you every time (In 2 games against the Stars, I left Jamie Benn wide open 3 times-he was 3/3)
-Chemistry/morale is still very meh. Not much has improved there.
-Trade logic looks like it may have improved, but again, I haven't spent much time playing.
I'm a franchise mode guy, so the addition of expansion mode is going to keep me entertained for enough time to justify buying the game, but if you're just looking for game play improvements, there's nothing really groundbreaking in 18 vs. 17, other than the fact that you can control your stick on D now, which doesn't really change the game much.
Probably not worth the $80 right now, but it's usually on sale at Christmas time for 50% off anyway.