This is a bit of a tough one; the biggest pet peeve I can think of from older stuff is probably with the writing and characterizations. In particular reading old Marvel books makes you realize that for all the fun ideas and cool heroes and villains... The female characters are really painful to read. (Go back and read the Wasp, Pepper Potts, or Sue Storm and see how awkwardly stereotypical they could be... It is a product of the time, but also very hard to ignore upon rereading now.)

A slighty more modern pet peeve that seems to have at least partially stopped is the "make them like the movie"-ification. Sure, Morrison's run was great - but I never liked the leather jacket uniform look for the X-men. Beefy emo Doc Ock was hot garbage, and organic web shooters can go straight to the dumpster.

As for a modern one that bugs me, it is the one that seems to have replaced the above; "Iconification" (for lack of a proper name) - where characters are regressed in characterization, design, setting, and/or supporting cast to make them line up the closest with the iconic versions that are known by the mainstream casual audiences and can easily be taken in.

As someone that grew up reading Spider-man comics, or DC books where characters would age, mature, and cast members would leave, die, or just be forgotten in favor of new folks coming in and out of our heroes lives - this recent trend frustrates the shit outta me.

Spider-man was my favorite super-hero book as a kid, and aside from mini-series and one offs I can't stand any of the books featuring him now. His personality is stunted, and he has an upsetting obsession with Mary Jane. He isn't a well adjusted adult who has been through hell and back and grown to be a confident man - he is a weiner who mopes about and stalks his ex and refuses to get on with his damn life.

Other books and characters get this too. DC had some of my favorite character in the "legacy" content - things like Wally West as the Flash, or the JSA mentoring a new group of young heroes, or Teen Titans being a team lead by adult Cyborg and Starfire - who now serve as teachers to the teens. (You read the part where Superman and Wonder Woman show up to tell them to shut the new Titans down and Starfire tells them to piss off and try to tell me that Starfire isn't a badass!)

This made the world feel lived in and we got to see our characters grow and become better people. Shit, Batman had a few character arcs that made his position really neat before DC "New 52"ed all over itself and ruined all that.

It is frustrating to see these characters that you have grown fond of and seen go through things get reset to a static point because that is the most friendly accessibly status for the brand. It hammers home to you that no matter how much effort and love the creators put into these books and stories, at the end of the day marketing and corporate interests are god. "Fuck you fanboy, we need to keep this as mass-appeal friendly as possible!"

Cause that sure is working out great. Look at all the sales jumps Marvel has had with the MCU bringing in new read- Oh right.