Bad for beta reading is not the same as bad for regular reading, for me. Things that would make me hit the back button in a posted fic are things to flag in a beta read. But bad for beta reading for me means . . . well, sometimes it's grammar, because bad grammar makes me want to weep, but more often it's just this indefinable not right feeling that's much, much harder to fix. And it takes a lot of work to pinpoint exactly where the problem is and help the writer work it out (if the writer is interested in working it out - some writers aren't in fandom, but that's a different issue).