-Elder Gore AI partner: Grandma is helping Grandpa out
-Penguin Mob: new squad-based gameplay!
-Emperor Penguin: this bird is tough as nails
-3 more Encyclopedia pages
For the HD version it means the following features in addition to the aforementioned:
Undead Jenny, White Gangster Gore, totems, kills instead of "score", kill multipliers, encyclopedia, hearts as health indicators, all kinds of optimizations, remade snow level, Elder Gores and of course wifi co-op multiplayer! The multiplayer works even between an iPhone and iPad :)