In Quake3 you have trilinear and anisotropic filtering and in UT you can run the high resolution S3TC textures on the GeForce boards.
Missing features matter, and they matter quite a bit in certain games. MDK2 was only the start. Evolva, Sacrifice and Giants look drool inducing on the GeForce and Radeon boards, nothing too special on anything else. Watch those screenshots for a while, now think of everything in the entire game being that much better on the GeForce board(the ground, water, characters) not to mention running much faster with T&L enabled.