This is the first time I've played Doom 3 on the PC and it looks a billion times better than it did on the Xbox. The level design also seems a fair bit different, though that's because the Xbox version was gimped to get it to work well on there. And I liked it a whole lot there, and I like it a whole lot here. I am happy with my purchase.
That said, if you already own Doom 1, 2, 3, and the Resurrection of Evil expansion, I don't know if $30 will sell you on it. It's got controller support for all three games, which is something that I don't feel mods could ever get done right. It's got the Lost Levels expansion and it's also got the previously XBLA exclusive Doom 2 episode, "No Rest For The Living". That's the new content as far as I can tell, and they said as much (though they curiously don't seem to have mentioned including that Doom 2 episode.) The graphics don't look tons better. The lighting is, indeed, a fair bit brighter, and ammo seems a bit more plentiful which always struck me as an odd decision - ammo was plentiful in the old Xbox version, too. The new shoulder-mounted flashlight works pretty well. Supposedly, id said that they were including the classic flashlight-gun switching mechanic, but I didn't find that option. I thought it was a cool mechanic in the Xbox version, and I still think it's a good idea, but if it really isn't there then this doesn't bother me too much. Note that when starting a new game I didn't look for the option - it may be in there.Motion blur has also been included, though you can turn it completely off.
EDIT: Also, a little feature, but it's probably worth mentioning that the gun sounds have been improved. The shotgun no longer sounds tinny, it's got a fair bit of a boom to it./EDIT
Anyway, some screens. My graphics card is only an HD 7770 so the resolution is 720p and I have anti-aliasing off, but the difference shouldn't be that huge.
If you have any other questions, just ask!
EDIT: I just uploaded a ton of screens and it doesn't seem to have included them.