Sourceforge was always hard for me to use, the standard template (code, forums, issues, mailing lists, wikis) were hard to set up (my experience) and use.
GitHub's UI really made impact on me, and the short name, the agenda from the people (I've seen couple of videos from the founders talking about not github, but GIT itself).
I still get very lost using git (p4 user at work, so svn feels closer) but with enough help from github I can see it done.
Visiting SourceForge feels like going to RapidShare, with all sorts of ads vying for your attention. Each project has numerous sections which may or may not have been filled out by the owner. Heaven forbid that they used the mailing list feature.
Or the mailing list archive software they use, which must be the worst of a bunch of very bad ones. It seems incredible that there is no halfway decent way to view mailing list archives (mailman's html pages don't count).
GitHub's UI really made impact on me, and the short name, the agenda from the people (I've seen couple of videos from the founders talking about not github, but GIT itself).
I still get very lost using git (p4 user at work, so svn feels closer) but with enough help from github I can see it done.
But all in all github is personal first.