There was just a question to the devs about TF2 and Overwatch. The devs pointed out that that would actually be a compliment to compare the game to TF because they love TF, but as you start playing Overwatch, you realize how different the two games are.
"Sure there are some similarities like objectives or game modes. We already have 12 heroes here in the demo, whereas TF has 9 classes, which are very customizable, so we are going in a different design direction. We also feature a lot of mobility in our game....
...Overwatch is about these amazing hero abilites, it's not about running around and putting people under your crosshairs....It will be fun to see players from others games like CS or CoD come and play, without using hero abilites....then you will start using abilites and that's when you will realize that this game is very different from the others out there"
Sticky request on post, since it's an official response to the most common question around.