Grammer/Spelling errors-Honestly, it's not that hard to type your diolouge into Word or something before putting it in.
Profanity for no reason - Sure at times swearing can add to a scene of some sort, but most of the time it's pointless and just makes the text laughable at how poor it is.
Bad Mapping/Scripting/Tiles - It's not that hard to make a decent map, if you can't make tiles don't, you should practice before you script.
Gamebreaking bugs - Surely you should be able to find a major, gamebreakng glitch. Including those is a sign of laziness.
Abandoning hacks - Good or not you should at least have the decency to finish it. If it's bad then you have experience of what not to do next time. I really hate it when a good hack is abandoned. Why? When you have so many supporting you, why stop?
Keeping stuff the same/minor edits - If you want to do a remake and want to keep the same maps fine, but if you just keep everything the same, theres no point. I'm sick of filling the pokedex, sick of walking through pallet town, sick of team rocket, sick of collecting badge, ect. If you make only minor edits though, that's far worse. For example, I've seen stuff like this before:
Exit house
Welcome to WATER TOWN
Me: Gee, these houses look a lot alike. Oh look! There's water to the south and grass to the north."
Walk into grass
BARK: Wait! Don't go out. You need a POKEMON to protect you. Here follow me.
Me:*Epic facepalm* *Shuts off emulator, deletes game*
If you do something like that, it just shows you're lazy and can't make the game different in any way. Isn't that the point of ROM hacks?
Overused ideas - This I really hate, more than almost anything.
Examples:
Multiple regions
4th and 5th gen Pokemon
Team Rocket returns
Prequel to Red
Remake of Gold
The list goes on.......