I tweeted just today that GitHub was the coolest thing out in the past <some unspecified bit of time>.
GitHub is ideal for the "fire and forget" method of open source. I can knock out a bit of code that might be useful to someone, throw it up there and forget about it. If someone likes it and wants to improve it they can go ahead and fork it without me ever having to think about it again. If I /do/ want to maintain it, it's really easy to keep track of the modifications people make and merge them back in to my version.
The amount of high quality code coming out of GitHub projects these days is astounding, and I'm willing to bet a LOT of it wouldn't ever have been released using the older systems like Sourceforge and Google code.
Not to contradict what you say about github (which is nice - I'm using it for Hecl these days), but as people like to say at the Apache Software Foundation, community is more important than code.
Obviously the best projects are the ones with proper maintainers actively working on the community side of things - but GitHub enables the community to take up a project even if the original creator just threw it over the wall.
(There's a lot more to GitHub than just enabling me to create irresponsible throw-away projects, but personally I've found that aspect of it to be very liberating)