So the question is what cool aspect of LiveJournal would be most fun, and most compelling, to talk about for five minutes. Judging from the Lightning Talks session I attended at YAPC, stuff like "here's a cool thing we implemented, or a cool technique we used" is more likely to be accepted than "here's this cool site we run".
Alternate suggestions are welcome, but I'm thinking syndication would be a cool talk. The friends view is syndication, and in fact I can't think of any other site that uses and encourages syndication as heavily as we do. Every single LiveJournal account can be used as a syndicated data feed, and about half of them actually are; is there anywhere else that has over 100000 channels that are actually watched?!
The technical aspects of this, and the social impact that it has on the LiveJournal community, are worth mentioning. We could also talk about what we're doing to encourage syndication of content between sites, how we're doing it, what modules we're using, etc. (And maybe that'll get me off my ass to write user-controlled syndication of outside content, and RDF export if it's not already running, before we actually give this talk. :-) )
Other thoughts? This is a new topic for me, so I don't know enough about what the rest of the world is doing with syndication; and I'm not completely dedicated to that being the topic of the talk.