Why Do People "Watch" Code? A Short Survey for Open Source Devs
phark.posterous.com
phark.posterous.com
What I'd like to see is a "★/☆" for a repo, branch, or commit. Similar to the way you can ★ a question or answer on StackOverflow.
I also watch projects that I think could amount to a big deal, but are currently in the concept phase. That way as they get going and start solving a problem I know how it goes.
Finally I watch projects that are solving problems I may have insight into -- so I see places I can contribute.
In all these cases I don't fork because I am not engaged enough to maintain an active fork, since they you must manually pull the upstream master regularly... annoying if I'm not doing it because I'm involved. Then I can just clone from the head easily when it is time to fork or modify the code.