Reznor: That's because it has to do scanlines, else some games which change parameters (say, for Mode 7) halfway down the screen can still work. I wouldn't argue with me on this, though. I've talked to PAGEFAULT about this.
And... another thing, PS2's GS is optimised for really small triangles. Radeons... aren't.
nephalim: Radeons (not the original, but 8500 and better) support pixel shaders 1.4 and vertex shaders 1.1. Radeons above 9500 support PS and VS 2.0. The difference with PS is the amount and types of instructions available... there's also a big precision difference (PS2.0 opens the doors to full floating-point precision). But realistically, games won't be using many 2.0 shaders for a little while, since most of the time you can still pull off equivalent effects with 1.2 (GeForce3, 4) and 1.4.