Things NHL 12 improved:
Sound effects. NHL 12 sounds more like real hockey in terms of stick and puck noises.
All centre-ices, bar Carolina, got wordmarks - even if they were just something to make them look less blank if the licensing was absent.
A few new Euro arenas.
The instant replay system and game highlights were much better.
Things NHL 12 downgraded from 11.
Custom music. Some of it doesn't work any longer.
Checking and hitting. Preferred it in 11 if truth be told.
Load times. Markedly worse in NHL 12.
Things NHL 12 ruined:
Stability. Offline gameplay freezes? Wholly unacceptable. A franchise killer for me.
Penalties. The sliders consistently work in 11, the refs WILL call you for a deliberate high stick, boarding, charging or interference and the CPU will take penalties according to your preference.
Goal celebrations. The crowd shot sucks the life out of it. When you go back to 11, it just feels so much better.
I can't fairly comment on the AI or BeAGM trade logic because I only played a small segment of a Euro season whilst waiting for the "mega-patch" and the roster update for 12. Obviously when the "mega-patch" fixed none of the issues that were making my experience with this year's title a frustrating chore, I went back to 11.