I miss the funky effects that could only be done in software, like making fur grow on cubes, lightshows, home-grown antialiasing, realtime halftone filters, voxels, etc. I also miss smoothness. Just about every game these days boasts about getting 100+ frames per seconds, but they still keep VSync turned off to make their benchmarks look good, and studdering and tearing are the result. I don't think anybody realizes that if you're getting framrates higher than your refresh rate, you're just wasting electricity and making your graphics look WORSE. If you're going that fast, you might as well turn on VSync and kill the tripple buffer. That's probably the only reason why I still think consoles can compete against PCs. PCs are all about raw numbers and features, but consoles are all about smooth motion. Ratchet and Clank for the PS2 still amazes the hell out of me, even though Need for Speed Underground and Far Cry on the PC have a million times as many special effects in their engines (plus, R&C is a lot more fun!) ;-)
A few things I really wish you saw more in demos are realtime fractals, and textures generated on the fly. 64K demos are still pretty amazing for making do with pure math, but you don't see mandelbrots spinning and zooming all over the screen. A pure, animated fractal demo would rock. I have an animated fractal for my screensaver (JuliaFractal), and it really amazes me how fast you can generate these objects on modern hardware.