Realism

Fatigue - finally a reason to turn off always run
Footstep/swimming noises
Stats determine physical ability (run speed, jump height, footstep volume)
Realistic behaving weapons - no "insta-bullets" (i.e. shotgun style)
Pick up ammo from missed shots, both yours and monster's
Burning fire - find water or "stop, drop, and roll" to extinguish

Items

Inventory based - use items when you want
Drop items/ammo for your friends
Keep up to 9 of each item from one level to the next
"Real" shields that have to be pointed at incoming attack
Buy items at shops (sell, too, in the final version)
Unique powerups, as well as classics (grapple)

Graphics

Each class has unique model and/or skin
Player models show active weapon, shields
Over 100 total new/altered models
Skins that show when target is wounded
6 completely new monsters models, including an all-new boss
15 new weapon models
Completely redesigned status bar

Levels

Over 24 unique levels (including towns and secret levels)
More than a 100 original new textures
Stunning, architechtually realistic design
Logical sequence of settings - levels flow together smoothly
Many detailed brushes, often interactive (not all just rectangle buttons)
Every level has a distinct and cositant theme