i would like to make videos and photos appear above my head for people around me to watch, and stream mp3s from my computer into my house for other people to listen to.
Yeah, music in houses would be spiffy! Other stuff though, especially in public, would be tricky -- I'd want to limit it to pre-vetted images/movies or maybe have it only visible to friends.
Otherwise you have to deal with the inevitable trauma caused by what the guys at kingdom of loathing lovingly refer to as the 'future of user-generated content = cock-and-balls' factor, which I've most recently seen at play when the awesome metafilter flockdraw eventually devolved into a days-long session of Griefers Repeatedly Drawing Goatse.... yeeeah.
That's where my mind goes whenever user-gen content is mentioned. KOL's big example was when they had to proofread all the bricks we made. Streaming copyrighted stuff may be a little on the illegal side, too. We don't need the RIAA raiding our glitch homes. ;)