Currently it's either overused to mean anything which is derivative of another source as opposed to specific references to rule_34, or underused since it's not on everything derivative of another source. Which should it be?

Rule 34 pretty much means that if someone's thought of it, there's porn of it. the tag started out meaning porn of anything odd and unusual (say, a lamp and a chair fucking), however, it does seem to be used more frequently with things that specifically reference TV shows, or movies or books. (as the tag info page says, pornographic images of a copywrite commercial character)... as in.. Mama Bear and Robby Sinclair screwing.

That said, pokemon +rule_34 has a lot FEWER results then pokemon -rule_34. Ditto for digimon. and Star_fox. so much so that it looks like they were specifically excluded from having rule_34 added to them.

However, adding either of these tags to rule_34 would increase in a rather drastic inflation of the rule_34 tag.

So.... I would propose that rule_34 is currently being used for things that may be particularly surprising, or unexpected. (Say, Link's Epona getting ridden hard by Seasame Street's Count)...

Anyone who spends any time in the furry community really won't be surprised by pikachu and charmander getting it on, or porn of Renamon or Krystal.(and then there's the puzzling slope of 'but this pikachu is MY pikachu character not pikachu) ... So... I would say, an it's NOT unexpected or surprising, anymore.