Overwatch is a well crafted game with highly varied character pool, interesting map dynamics, fresh visuals and sound. It's quite incredible how well Blizzard managed to balance the game at start given the variety of hero playstyles, but well.. they have been doing so for other titles for quite some time now.
The game is 6v6 experience with high focus on team composition, tactical choices and cooperation. Each hero has a weapon, a few abilities and excels at certain aspect of the gameplay. To balance that s/he is often hard-countered by another hero's ability or play-style. Ultimate abilities are often so powerful they can hardly be countered at all. This is why teamplay is very important as some squishier characters have hard time surviving on their own.
The amazing effort put in gameplay design is partially offset by a lack of appealing reward system. The "leveling" has no effect on the game other then unlocking cosmetics. The achievement system is more suited for a single player game requiring to complete a difficult task only once. The algorithm behind "play of the game" highlights can be easily exploited by certain heroes; while others have no chance of getting one. There are also no incentives for an individual without a group of friends to cooperate with others or exploit existing synergies.
If this wasn't Blizzard title Overwatch could quickly follow the fate of Evolve - beautiful, fresh, well-crafted game without long-lasting appeal. For sure, Blizzard will promote Overwatch to be played competitively, and most likely at that skill and cooperation level this game is immense fun. I fear, however, that casual players will abandon the game quite quickly if no drastic change to reward system (or game modes) is implemented soon.