Another feature all computers games should have is a demo feature. There were some pre-cursors before, but this feature really took off with first person shooters such as Quake. Turn on demo mode in Quake or Unreal Tournament or Tribes and the game records the action onscreen to a file so the users can go back and view the action again. For most games there are editors available so you can go into these files and extract particular parts so you could make a greatest hits demo. There is even a whole subculture of folks who use the demo features to make movies.
I was pleasantly surprised that Age of Empires II also has a demo feature, and I wish more computer game companies would include such a feature.