Watch Dogs is one of my favorite PS4 games at the moment. I really enjoy playing every aspect of the game. But the rest of the world doesn't agree. As mentioned by HeatherGrace, the marketing for the game suggested that it would be of much higher visual quality than the final product could ever live up to (it still looks stunning though).
A big issue is that it's a current gen game that runs on previous gen. Therefore the PS3 and Xbox 360 versions were probably not that impressive. On the other hand, the direct competitor; GTAV, manages to look beautiful on previous gen hardware, and a little better on current gen. Another thing going for GTA is it's variety in world designs, customisation options, and vehicle attributes.
Watch Dogs has some interesting new ideas, and a pretty gripping plot. It not living up to it's full potential just makes me even more interested in a sequel… after all, I feel the game does stand fine on it's own, as a person who merely stumbled across it, knowing nothing beforehand.