I noticed that too. Adding tags, getting views, and getting comments all seem to add up to interestingness. It must be more than that but it does something.
it's a pretty complicated algorythm... based on views, vs. time, factoring in groups posted to, members of those groups, comments, contacts, complete strangers, favorites, etc.
Mine went from least interesting to number 24th most interesting in my photostream by simply posting it here. Lol. I wonder if the flickr algorithym isnt just some random generator with a couple of filters.