Like most people have already stated, I too hated the cheap teleport-in "boo!" scare tactic of Doom3 monsters, it was too predictable and didn't make for exciting gameplay.
Bring in the fast-paced, almost frantic gameplay of the original Doom/Doom2 games.
The setting needs to change to oldschool hell mode, you know the thrash metal "slayer" style hell with over-the-top flying cacodemons and pentagrams and shit.
And of course the most important thing, the BFG.
The original BFG is simply what seperates Doom2's AMAZING FPS deathmatch experience from anything else. Those who have played Doom2 multiplayer will know what Im talking about, the gameplay mechanic behind the weapon is unique to Doom2 and has never been reproduced since.
I have a nifty blue line!