Printing. I have games, I want to print them. I've tried PGN readers, and determined that most of them suck. ChessBase doesn't suck, but it's Windowz only (I happen to use Windowz, but maybe some people don't?), and it costs enough to feed a small 3rd world country for a year.
I know it's not a terribly 'core' feature, but it would be awesome.