Bungie's level, hacked down to playable net size. Still pretty big... my guess is 4 would feel lonely.
Again, Bungie's level cut down to net-playable size. Lots of nooks and crannies... just like the original. Only this time, it's your friends you need to worry about, not hunters...
The trench around the arena in House of Pain has been filled with water.
A few tweaks to the first room of that level, but basically it's just a big, open arena with the sniper level far, far away from the pit.