Advanced 2D engine with shader support
Particles with giblet effects
Lighting
Tile system
Animation system
Improved combat/gameplay systems (player will actually move the level instead of auto-scrolling)
The return of survival mode!
Blood, blood, blood, and some gore (as well as corpses not just vanishing!)
Item shop
New and improved zombies (and different types as well!)
Bosses
Aliens!
New weapons (you know you want a flame thrower!)