I'd have to agree with vimbert on that, in that anything people do or would likely tag "tragedy" would just be some adolescent interpretation of dark & edgy
"sad ending", rather than anything remotely resembling legitimate literary tragedy. >>4654
As far as the overall use and obsession with tags in fanfiction versus regular fiction, the reason is pretty simply of course; people picking up a book know what they're getting into - they're starting to read it because they know the author, or the genre, or the topic of it in advance. Whereas fanfiction does not have that luxury. And people tend to read a lot more
fanfics in a span of time, given their short and usually easily digestible nature. And with the glut of fanfics in any given genre, tags are just the natural result of wanting some way to filter the type of things readers are interested in and connect authors to their target readership, in a medium where there is very little name recognition (save for a small handful of authors) and little to no way otherwise to know in advance what genre a story will be.