This post is inspired by Eva's thoughts on getting scientists to adopt Web 2.0 and Cameron's post on making Connotea a killer app for scientists.
Many people have added their CiteULike or Connotea libraries to FriendFeed, so during the day you can see various new papers flow by. Similarly, journal's TOC updates and saved searches on PubMed create a regular stream of possibly interesting papers. Lastly, after a few weeks or months, papers are processed by ISI Web of Science and can be tracked by citation alerts. In the end, you might see the same paper flow by a couple times.
This situation is far from ideal. You see echos of the same paper and papers arrive via multiple channels: RSS, email, web sites. There are far too many potentially interesting papers, so you have to focus your various alerts in order not to be overwhelmed.
My proposal for the killer app is a central place which tracks all of the above items (i.e., friend's libraries, PubMed searches, journal TOC and citation alerts) and integrates with your personal library. Just like in FriendFeed, there should be a way to rate/like a paper ("Faculty of 1,000,000"?), to prioritize the new papers, and to save papers to your library. The most important and difficult feature would be to merge equivalent entries, i.e. a Connotea link to PubMed needs to be merged with the journal TOC alert etc. So when you already identified something as interesting and filed it, you won't be alerted again if it comes in via another channel.
Of course, there should be a non-mandatory way to tag papers, to have groups, and to recommend papers to specific users (like the "for:" tag in delicious.com).
Bonus points: keep track of comments and blog posts of the paper, plus all the extended literature analysis that Cameron proposed.