I think it would be very useful for people to find manga from Fakku's now enormous catalog of manga if people could sort it by category.
Examples:
Incest, Testicals, Gender Change, Mind Control.
And since multiple tags could be applied a manga tagged with incest could also be tagged with:
Sister, Father, Brother, etc.
Its probably hard to implement, but I'm sure it would be very much loved by regulars :)