With Doom 3, Carmack was all about bringing the next generation of lighting improvements, to take the envisioned game's immersion to all-new levels not yet achieved anywhere else. An attitude seen many year earlier with Quake II. In both cases, he absolutely succeeded. Back at the time of original release, the lighting tech was a real shock to the system. I haven't played Doom 3 since those original years, but I imagine that lighting tech still stands quite strongly.
I've just got myself a new (old) console so I'm in the midst of testing games with that, along with some NFS2SE on my 3dfx'd K6/2 (playing along with the OCAU dudes), and now up to the final few levels of The Lost Vikings also on the K6/2. I'm not a very skilled gamer - I simply don't dedicate enough time to Actually Playing - so the difficulty is reaching the stage where it's almost not fun any more, and I potentially may not finish. Which is not a concern of mine, I have no ego when it comes to completing games.