Intentionally Prolonging a Game (aka: Kicking The Losing Team When They're Down)
A trend I've seen far too often - typically in ARAM - is the tendency for a winning team to intentionally drag out a game just so they can beat further on the losing team. It's infuriating, and gets me to the point where I simply don't want to play.
And for clarification, I'm not talking about the "Someone just spawned, let's kill them and THEN end", I'm talking the "All of the enemies are dead - let's just stand here and /dance or /laugh spam to rub it in their faces!"
The real-world equivalent of how this makes the losing team feel would be like the winning team spitting on the shoes of the losing team after a freaking Little League game.