What I like in Overwatch, but not in TF2:
It's solid. Everything is tested thoroughly, gameplay map, modes, nothing breaks almost ever, even with crazy combinations of abilities it's still rock solid on every corner. In comparison, TF2 feels like half-assed device that's hold up by many thin strings all tangled up together and some duct tape.
What I like in TF2, not it Overwatch:
Freedom. It doesn't feel constrained, both in gameplay and expression. You can rocket jump at what speed you can reach, have fine air control - movement is as fast as your reflexes. In Overwatch you have a huge rubber band tied to you back - do something more and you'll be pulled back. All the timers feel so artificial, and movement stuff, like Phara's conc jump, that for some reason stops you after flying 10 meters, is infuriating.
The same with expression. While OW has a bigger selection of characters, most of them feel generic and doesn't allow for customization. A lot of people bash TF2 for it's skins, but I think it added a new life to it and new ways for people to play. Skins, items, taunts, a myriad of voice lines led to some most fun and hilarious online interactions I've ever had.
"The most fun you can have online" is still true about TF2.
Too bad the game has been dying for several years now. It feels like Valve has no interest in it and is constantly waiting for it to die, but it somehow just doesn't die - it's in limbo state where there's no point of investing in it any more, but killing it completely isn't an option either.