Pretty much as you mentioned, jokes that have been done to death. Usually the 'relatable' ones
"6am, I am going to get things done today. later at 12pm nothing is done. I'll try again tomorrow."
"I have a week to finish this project. I have all the time in the world. netflix and twitter later *person now rushing against 2 hour deadline"
"Artist is drawing a picture with a pencil, moves hand to try and 'ctrl alt delete' remembers they are not working on the computer. Noooooooooooooooooooooooooooooooooooooooooooooooo"
In addition to that. Comics that incorporate/recreate memes, such as.
"Character sitting in a house that's on fire with smile on their face. 'This is fine."
I mean those are fun in their own way but keep those on your twitter or something cause redrawn memes 75% of the time feel like fakeouts or wasted updates when I'm subbed to gag comics on tapastic
Inside jokes are also pretty bad. Why... just... WHY bother making a comic public if only your close friends are going to get the joke and the rest of the world is going to be confused?
Cross overs into licenced universes. Don't get me wrong, sometimes these can be pretty gold, I'm talking more about the ones that just start to feel like a fanfic to the point you're wondering where the jokes in the gag comic disappeared to (I don't know why but Harry Potter is one that has happened a few times, where they'll do a strip with a Harry Potter themed joke and then the next 20 episodes will all be Harry Potter jokes... and not even jokes, just characters doing stupid obvious shit that you know is stupid obvious mistakes if you know that series so it's not funny because you know how this will turn out. Another sin is just... inserting a licence character... as a permanent character???