These are my irks. Some of which are the same as others who have already posted to this thread.
Garbage in caches/inappropriate items - rocks, bottle caps, food items
Cache owners not maintaining their caches despite several DNF's and/or a maintenance flag.
Container size/terrain rating inaccurate. Then, cache owner not even updating cache page after a finder let's owner know.
Poor cache placements — 20 feet in front of a busy store entrance, on a busy road where there's no sidewalk, right next to someone's residence (despite it "technically" being on public property)
Missing/stolen geocoins
Useless hints
People who take a swag item and leave nothing.
People who stop caching/move out of the area and don’t adopt out their caches or archive them. The caches usually end of deteriorating to the point where someone eventually flags a “needs archived”.
Cache owners placing regular/large size caches and put 2 small swag items - Kind of pointless. May as place a small cache instead.
People who cheat - Logging as found when a find was not made, asking for puzzle solutions, etc.