It can be anything! Obviously we're all different so different things annoy us!
For me, I find incorrectly sized clips an annoyance - must be the OCD - my eyes keep flicking to the sides or tops of the screen where there's black space!
And also lip flapping - where the character's mouthes move but the clip is muted so they're just sort of going blahblahblah - looks messy to me !
Also enormous watermarks. Or even just watermarks. If a thief gonna thieve, they'll take it watermark or not.
I bet we've all got some weird and wonderful ones! GO!
(fyi this is about actual editing not about shit that youtubers do lol)