Most people here have mentioned my peeves, so I'll add some I haven't seen.
-I don't know about anyone else, but I personally can't stand excessive swearing or F bombs in comics, especially F-Bombs. I just feel like there's always another way to get the point across. Unless your character is actually like that, like a mafioso type; something I'm trying to figure out what I'll do about.
It's strange because I'm not really against swearing in life, but it always reminds me of the old subbed anime groups that made characters like Chopper from One Piece or Sakura from Naruto ask, "What the F*CK are you doing?!" It just comes off really overly edgy and try-hard.
-I really don't like when characters are really overly hard boiled and angry ALL the time. I really only find this in western comics (part of why I don't really like them all that much)
I hate to name names, but I found this in a particular comic series that a TELLTALE game is based on where EVERY SINGLE character in the first 2 graphic novels are SO angry and pissed and grouchy that they also drop lame F-Bombs almost every other page and they all blend together because they do this. I was so excited too, since the game did such an amazing job at differentiating its characters. Needless to say, I met the author/artist last year and he seemed really nice and even asked me to sign an old cover project based on his comic that I did. I could see him being like those characters, but I guess I'm ranting now...