There was some article on the intertubes about how Github panders to devs, while Sourceforge panders to distributers.
Mainly I would say that it works great for projects with relatively few contributors (like most Ruby gems), but seems to fall apart on very large projects (like Rails, who uses Lighthouse for issue tracking). Everything else about Github is amazing, though.
dhh doesn't sound like he was ever a fan of Lighthouse: https://twitter.com/#!/dhh/status/63366853636009985