man, I must have spent a whole summer in halo 2 superbouncing and rocketlauncher-sword zooming (I forget what we called that) to see where I could get on maps. I hated it when people did that in ranked matches though.
I think we do these things because it's a new way to play a familiar game. Games get old and new games can be fun - especially when all your friends are already playing them - and you're tired of the "real" game.
Take a game like halo: standard shoot-em-up. Now you're practically free-running with superhero-like glitches flying you around the map juuuuust short of that one ledge. *runs around and tries it again*
It's just a matter of time before someone mentions machinima. Machinima is awesome. Most noteable is Red vs. Blue (another halo ref, I'm sorry) - a fan machinima that's been going for YEARS and even got it's most recent full-length film put into CANON. I couldn't believe that when I heard it. You know what, I just tried to get a citation and couldn't (going off of what my friend said). Who can confirm/deny that claim? I'd appreciate it.
Tricks and machinima. I think game developers (at least at Bethesda(spelling) and Rockstar games) are quickly realizing that sandboxy games are an easy way to let the gamers' imaginations add 100s of playing hours.