Dungeon mastering ain't easy, and anybody who says it is either uses store bought adventures in their campaigns or is playing with a bunch of kids who can't tell a gelatinous cube from a bowl of Jell-O.
But the DMs life is made even harder by players who refuse to get with the program, players who seem to think it's fun to bog down the game with their "creative" moves that are nothing but flashy stunts:
I had a player who kept making dumb technical arguments to justify every little weird stunt he wanted to pull. Ran out of arrows and wanted to shoot rocks with his bows, kept arguing that he'd seen someone do it so it totally worked.
Couldn't make camp in the muddy grimey floor of the cave so he wanted to sleep on the ceiling upside down with his boots of spiderclimbing. When we pointed out that the blood'd rush to his head, he argued that there wasn't a sourcebook that stated that elven biology was affected by gravity that way.
Sooooo, a goblin ran around the corner and threw its poop at him, he exploded and we pointed out that there wasn't a sourcebook that stated goblin poop didn't double as volatile explosives when thrown at upside down sleeping elves.
Then there are those players who try to get super tricky with every move they make, thinking they can outwit the DM by coming up with some super clever plan:
I once had a guy that tried to cheese the big boss fights, saying stuff like "there is nothing in the rule book that says dragons can dig, so im going to make a bunker and blast it with firebolt for 3 (indame) weeks". (for the record he was a lvl 1 character wanting to do this solo. The dragon was meant to be fought by 3-4 lvl 4 players)
True, the rules dont say dragons can dig. But Im the DM and I say the dragon can and will rip through your mud shack and violently disassemble your rib-cage.
But to me the worst RPG players are those who use their character as a way to reveal their inner demons and live out their own dark fantasies, because their in-game actions reveal they may be a real life threat but you can't really do anything about it. What are you gonna do- call the cops and tell them a player character has been raping NPCs?:
He kept trying to rape female NPCs and female player characters in game. Saying that it was "in character" for him. He found a Scarab of Death later that session and after it burrowed into his heart I politely asked him to leave.