With BitTorrent and everything else, logically everone just has a network around a single file everyone is interested in. It makes tit-for-tat really practical and quite easy. Almost like darwinian natural selection (well, evolution is like moore’s law, you can just about apply it to everything imaginable ignoring the massive pardigm tangents that result), the popular files are fast and the unpopular ones get slow and/or eventually die, totally lost.
Certainly this almost capitalistic approach is efficient for the massively popular files where the chances are that you will encounter many fast seeds. But for those less fortunate, the downloads may slow to a crawl as peers may be geographically distant.
What about something like freenet, where everyone has a cache of data that they may or may not be interested in. With such a massive netowrk, you no longer have to worry about trackers taking over as all of them are now on equal ground, finding trackers is useless as they are inhrently found. Leeching is solved by a global karma system, and all of the other problems are replaced with the single complicated problem of scalability (which totally makes the whole concept crap).