Actually, pretty much anything.. Is the ability to customize a game either through the use of a map creation tool either in game or out (like LBP or Quake as being either side of that) or mods like in WoW or Dragon Age (although DA's tool is more than mods)..
You can also go as far as Neverwinter Nights (that the D&D one that was user driven right?) and stuff like Civ.. Do we think Civ would have been as popular and lasting as it has been without the user community bringing fresh content and even full-game mods? What about WoW? Was it the ability to modify your UI in an unprecedented way one of the reasons that it has done so well, or just a happy coincidence?
Character customization like being able to change what your character looks like in the game through ingame mechanics isn't exactly what I had in mind tho. unless users can REALLY customize them maybe like the cars in Forza (I think that was the one)
Really, these questions are going to be short (given that they originate from twitter), so answers can be free-form in interpretation. You could always give a : if you mean this, then... but if you mean this, then.. The forum gives a lot more freedom in answer length than twitter affords.