I attained a copy today, and went through some stuff.
Apparently, dead bodies and decals eventually disappear after a few seconds.
Weapons:
All of the old weapons, sans the Combat shotgun, are back. There's also a soul cube and a machinegun, and the chaingun packs quite a punch. The berserk is back, too and can kill most monsters in a single blow. You also run faster with it. You also get a flashlight for dark areas, but you can also bonk monsters over the head with it.
Monsters:
The Arachnatron, Baron of Hell, and Spider Mastermind have all been cut. The new Imps are sneaky bitches. The game's final boss is supposedly the Cyberdemon, but I haven't gotten far enough to make a fair assessment. As expected, the Cacos are back. Whether or not they double as a Pain Elemental is unknown, I killed the first one I saw too fast to find out.
Items:
Gone are any artifacts as far as I can tell. Only the standard Armor, shards, health kits, and berserk are available.