some of the problems are that it excludes ppl who are blocking googlebots, and will see things that aren't actually listed as intrests...
I wonder if it would be possible to have something load http://www.livejournal.com/interests.bml?int=interest for as many interests as input, search for every instance of http://www.livejournal.com/users/usernameshere/, make a list of usernameheres (with an instance in the list for each page it's on, obviously), sort by number of instances in the list, and cut off anything below 2 (or a user-specified number) of instances... then simply display in order by number of instances, looking like:
instances | username
7 | bob_the_great
7 | robby_san
6 | discoleetman
2 | randomusername
(only with a table instead of lame | separators?) maybe show which interests they showed, although that would be a bit more work...