The top three things that ruined an otherwise great game for me:
Navigating indoors, especially stairwells, feels like your character was crossbred with a Buick. Overall movement was clunky and in many cases deadlier (and more frustrating) than the zombies themselves.
Zombie AI ignoring walls and running through them.
Hackers. Not only your typical cheats where people start off with the best weapons and god mode (although it was abundant when I played), but really crazy shit like battleships sitting on top of cities, artillery shells flying around, and every player teleporting into the clouds only to drop to their deaths.
The only way to really enjoy DayZ was to hook up with friends on a forum who own a private server.