Which threads or team efforts fall by the wayside without exposure? Or would create lots of duplication? Or would carry on without Macrumors guidance since there's no way of tracking progress?
The Song of the Day probably would - since we'd end up with hundreds of single threads.
Nike+ does since it's only when it's on the first page of Community that we see new entrants - and without new faces, the challenges can be less fun.
Folding does - since even though there's a Distributed Computing subforum, you need to know what it is before you can entice people in.
Losing weight hasn't been so much of a success. Perhaps because people are wary of posting actual figures in there - or pictures. Same goes for Push-ups to a certain extent. There's nothing really tangible to show for the effort - like outside tracking of progress.
Giving blood - while a most excellent cause that I try to do at least semi-regularly - is more of an community advertisement. There's enough offline and official marketing of local events and the supply isn't going to dry up if MR members don't know about it. Hmm... perhaps there's a new product. The Needle+ to attach to your Nano and count how many pints you've given? 😉