This guy has been working on this project for over two years since I subbed to it and holy crap.
Custom modelling out the wazoo, more command blocks than should be possible to fit in one world, but there's no mods in sight. I just hope that, being 1.8 based, the command block lag isn't too bad for me. I've heard upping your allocated RAM and lowering render distance can help that.
Seriously, I've seen minigames built in Minecraft. Things like Space Invaders or Peggle. Never full scale GAMES. Is Minecraft even a game anymore or closer to a game-building platform?