... Map Editor
I am thinking Doom, Duke Nukem 3D, Quake, Unreal Tournament. There were litterally thousands of maps made by community. Some were badly made, other very original. I remember playing an unreal map that was a giant bathroom.
Now, maps are sold separetly by pack of 4. This is what drive me away from modern multiplayer FPS. When will we see a courageous publisher ginving us a AAA FPS suchas call of duty or Battlefield, with a powerful map editor and let the community create thousand of maps, if anything at least as unranked game. Then the publisher could review, refine, balance and add some of the most popular map in their ranked match.