1) Allow people to see replays of games. Players can opt out if they don't care to partake in this.
2) Allow people to observe games live (but disable chat). Some potential for abuse, here, but I think otherwise good for the community.
3) Allow users to choose what colours they want their pieces and their opponents' to be. Some people choose colours which are pretty obnoxious to my eyes. They may love it, but I don't need the additional headache of clashing colours.