The design of ArenaNet's GW/GW2 has always impressed me. 99.4% of all of the game is contained in one file: GW2.DAT, which is (currently) 66.7 GB. I/O for updates is extremely efficient.

I've seen many games create a vast file system mess, and they're forever plagued by "corruption" or something, complete with "fix" tools, "cache" clearing push-ups and players reinstalling after big updates that don't install correctly for whatever reason. Somehow ArenaNet surrounded this whole problem and solved it optimally.