There's also all the basic stuff that defines a badfic - characters being OOC, spelling/grammar issues, ignoring the rules of the world, manufacturing tension ("Oh Legolas, our manly man-love is forbidden because of prejudice which there is no hint actually exists in Middle-earth!" ~ No-one, ever*)... heck, bad slash can have a Sue or Stu as the main character (remembering that femslash exists). And then there's the general-purpose badly-written-romance stuff ("Oh Leggy my darling will you forsake everything to be with me?" "Yes, dearest Saruman, I shall! Now let me climb your dark tower, if you know what I mean." "Orthanc? But there's nothing up there but the roof..."), and just... it being a bad story. ;)

hS

*How do I know that indicates it's not a thing, rather than that it just never comes up? Simple: Bilbo Baggins, noted crazy person who wandered off with a bunch of foreigners and came back inexplicably wealthy (ifyouknowwhatImean), and also a bachelor in a society where massive families are the expected norm, is allowed to adopt his young cousin with absolutely no fuss, so they can live together in a huuuuge empty house. If people were fussed about homosexuality? That would have been the subject of their rumours, not 'he talks to elves sometimes you know'.

(Note: no, Bilbo and Frodo weren't lovers. But the good folk of the Shire apparently wouldn't have cared if they were)