The game was slowly chiseled away from a strategy game into a macro action game.
Maps are all the same and boring. All the fun stuff maps used to have are gone. Maps even hardly have gold bases anymore.
Many of the things that made the game fun were changed out of the game.
The game rewarded mechanics, formulaic play, and macro too much over other things.
Not enough room for players to express their individual style.
New top players too uncommon.
The community around balancing the game was dogshit. I mean some 30% of the discussion was just fucking losers trolling and flaming a thread to death or people who didn't understand the game adding their two cents.
Little to no "high" moments in games. A player now wins by slowly macro-ing their opponent to death.
Part of the community didn't like what SC2 was turning into over time (e.g. DAY9).
Little new blood coming into the game.
Too many frustrating moments in the game for players (e.g. dts, old fungal growth, etc.)
Many balance changes turned core game elements on their head (e.g. protoss now being the aggressive expander instead of terran in pvt)
Not enough support from Blizzard.
Battles too fast.
Deathballs being too common.
Many silly things in the game that should have been removed or changed sooner (e.g. pylon overcharge, locusts, reapers)
Monetization for players is very skewed.
Some game elements limited options of the opponent too much (e.g. creep + overlords)