I'm developing a habit out of "finish the game almost up to last boss fight, then abandon it for a month, and then come back and finish the boss fight with cheats cuz you're too lazy and impatient to do it properly". Doom 3's add-on pack Resurrection of Evil is the latest in the trend.
It follows pretty much the same path as Doom 3. Go down corridors, hallways and occasional open space, kill monsters. Some innovations though. In the weapons space, there's Grabber. It lets you grab stuff and toss it around. "Stuff" includes random artefacts like rocks, smaller enemies and projectiles from larger enemies. This was actually pretty neat -- one of the more annoying characters in Doom 3 were the flying babies/faces. Here, you can just grab them with Grabber and be done with it. Plus, the zombies that throw fireballs and plasmaballs at you fall prey to their own stuff -- you can grab their projectile and launch it right back at themselves. Once you get it, its great fun.
Another innovation for all the big bang fans is the double-barreled shotgun. Just more punch in their face.
Third one, the "artefact", which was Soul Cube in Doom 3, is back but also different. The effects it provides are slowdown and invulnerability, which are both quite handy at times, and there are some puzzles you can only complete with slowdown (like walking through some punching beams which go at a fast pace). Also I don't think there was walking in sewer galleries with a biosuit in the original version.
Don't find the replay value of these things too high though -- I doubt I'd bother to do D3 or Resurrection of Evil more than once. The corridors were a bit too dull and repetitive for that -- I like open spaces and open paths more, like in Far Cry or some WW2 shooters.