Overwatch is a sci-fi, team-based, first person combat game that focuses on team composition and strategy more than (although not to the exclusion of) individual skill. While it is often compared to Team Fortress, I feel that it is more of a evolution of the DOTA and League of Legends crowd. Every character has their own unique gameplay, cool-down powers, ultimate abilities, and counters. Knowing the composition of the other team and changing characters as necessary through the match is essential to success.
The game also builds in an addictive reward system that does not change the balance of the game, but still drives players to get in "just one more match." Add in event-themed rewards (including character skins, sounds, and animations) and modes, and this game continually gives reasons for players to come back again and again.
I don't know how long my addiction to this game will last, but I've had a great time with it so far and highly recommend it to anyone who plays action video games.