I just played the first 10 minutes of it. Could be spoilers of the first 10 minutes, but nothing to earth-shattering happens so it probably won't really ruin much plot.
Basically, UAC re-opens the facility, sends in some marines to the lower levels, you (a new Doomguy, not D3's Doomguy) picks up a new artifact and re-opens the gates to hell.
Everyone dies, etc. etc., you walk around in dark cluttered rooms fending off enemies that teleport in behind you over and over again. As usual, you spend a lot of time squinting in the dark with a tiny flashlight.
So.. basically, more of the same stuff. If you liked D3's gameplay, then you'll like the expansion's. I was kind of dissapointed, was hoping that they'd have made things a bit easier on the eyes and toned down the contant baddie teleporting. Its just not fun. If Doom1/2's enemies all teleported in, think of how much it would have sucked.
It'll probably just collect dust on my desk until the
Last Man Standing mod team adds coop support for the expansion's campaign. No matter how bad the game is, it can instantly be made fun again with a couple buddies and some beer. Daikatana was even a blast to play.
Oh yeah, and the HL2 grav gun is in, and you get it within the first minute or so. It can pick up physics objects, enemy projectiles (imp balls) and also it can pick up some small enemies (lost souls, not impies).
Functions differently from HL2's HL2 grav gun. D3's one can only pick up an object for limited time, depending on size. Hold down fire, when it senses an object thats good, it pick it up. Release fire button, shoots object. Press reload to drop object.