As a Medic main in competitive 6v6 TF2, Overwatch is completely different from TF2 and should not be compared, period.
TF2 is indeed a team based shooter with the same kind of style in terms of game modes and how they're trying to push the Overwatch lore rn, however the games are just...
It's just different. TF2 has more skill based DM while Overwatch is more tactical and I'd have to argue it is a LOT more team based. All heroes have their specific counters. In TF2, sure the classes are built so that not one of them is stronger than the other, but like... the flow of the game is completely different because the only 'ult' is medic's Uber. A scout with good DM, or a Demo who knows how to move well, or a Soldier that can fly can dominate a team in TF2. In this sense, I think TF2 is more of a solo game, while there is teamwork involved, and Overwatch is so much more team based because of the restrictions of the characters. And of course there's also the skills & ults that everyone gets to tip the scales.
In Overwatch... it's really hard to push without your team; sometimes impossible to do solo antics. And even if the games feel faster--TF2 plays a lot faster than Overwatch.
But of course there are similarities. I still compare Pharah to Soldier, Junkrat to Demoman, Torbjoern to Engineer, Mercy to Medic, etc... but in essence I believe these are watered down versions of them. Pharah is not as mobile as Soldier, that's for sure.
But of course it's an opinion. I don't think it's TF3. I think it's its own game. Also I've edited this several times because I'm at work and I'm PROBABLY not making sense on what I'm talking about, so :P