Just a quickie here -- why don't more games have the "game within a game" mentality? Games like Counterstrike and Warcraft III have a seemingly endless replayability to them due to the amount of mods that people make for them. Hell, I was playing Defense of the Ancients this morning, and there were still over 100 people hosting for a game that's over 5 years old!
I really wish that more games would take advantage of an open mod/film system. Can you imagine screwing around more with a game like Battlefield? Or what about World of Warcraft people making their own shit? Another game within a game, where people bet on slotcar racing (yes, this is dumb, but possible)?
I just wish more people would imagine the possibilities.