For me it was the complete lack of "depth" of gameplay.
I hate to make the obvious comparison but when side-by-side with a game like TF2 the shallow nature of Overwatch gameplay really shows.
For instance, in Overwatch, Pharah flies to out herself in advantageous positions, this vertical lift is achived by simply pressing a button. To achive a similar effect as soldier in TF2 you must learn how to rocket jump, air-strafe and be able to pull the manuver off in a fast enough manner to get shots on target. This is only one example of the absence of any kind of substance to the gameplay, which could be described by the phrase "a mile wide but an inch deep".