Hrm... I have a few from both sides of the creating table.
As a creator, my biggest annoyance is backgrounds, specifically cities. They have to be specific enough, with enough detail that you recognize where you are, but not so detailed that they take away from the characters engaging with each other. Its worse with cities because they require so much more detail in a "designed" way; fields don't need that much detail to convey that they are fields, underwater environments can be conveyed with just colors if you're skilled enough, and forests/jungles are so organic that the lines don't have to line up. Cities are different because they are angular by design, more often than not (can't usually get away with curvy loopy buildings), and everything must have detail on some level. So yeah, cities are not my favorite thing.
As a reader, I HATE when comics devolve into talking heads. Even worse when all that is depicted is a 3/4 view head, just standing flat, with arms at the side spouting off a wall of text. It gets worse when a creator thinks they're being clever by tilting the camera to mess with the composition, when the characters are still standing at attention as they talk to each other. Comics ARE A VISUAL MEDIUM. Sure, not every pose has to come out of Pixar/Looney Tunes, but a lot can be conveyed in even the slightest hunch, limp, slouch or crouch. Composition plays a big role in the subconscious analysis of the scene (even if that is just the camera hovering behind one character over another; now we know who we're supposed to empathize with!), and giving two characters equal space in a neutral perspective when the scene DOESN'T call for it just screams of laziness.
And for walls of text, I'll read an essay or a prose story if I wanted to read a wall of text! Flashbacks are annoying but they are preferable to having a character tell us things we then have to do the extra leg-work of imagining in our heads. Comics are a visual medium; if you have the opportunity to tell your story with visuals, PICK VISUALS EVERY TIME. SHOW, DON'T TELL.
But yeah, those are my major peeves, both of myself and others.