"Unfun"

I've been hearing this word tossed around a lot recently. Everything from 's stun to 's poke to 's binding, it's all "unfun". Why? Can somebody please define the word for me?

In a game where the entire objective is to gain as great of an advantage over your enemy as possible, ganking, CCing and splitpushing your way to victory, who decides what's fun? And for that matter, who says what's fun both to play as and to play against? probably enjoys hopping around the map deleting all in her purview. No matter who you're playing, being on the receiving end of that isn't fun. and giggle away as your team pushes down mid while they whack your turret before you can reach it. Having to pull away from actually enjoying the game just to go and play tower defense several times probably isn't fun either.

So where do we draw the line at what's healthy to play against and what simply makes playing a frustration? Is "fun" really a factor that should be the main issue when it comes to balance? And if it is, why are only certain champions and aspects of gameplay adjusted by it?