mathsadi don't know why people are painting this game as a replacement for tf2 lol.
It's because they're marketing it really hard at TF2 players, and to get to the top people will probably have to commit to one over the other given that in reality the games will be pretty different.
Aesthetically it looks like the usual Blizzard clusterfuck of influences with no strong unifying theme (possibly putting them into the territory of straight rip offs but that's another conversation). Not many gamers care about that kind of thing but Valve unquestionably do aesthetics better.
The main point of departure is that TF2 is an inherently very high individual skill environment and most of the advanced movement (and probably attack) options in Overwatch look to be push button.
IIRC Robin said that one of the points of supporting streaming in client was to make players want to emulate and learn from highly skilled players, so they'll be playing to the strengths of the game engine vs Overwatch in some respects, assuming it materialises.