The respawn shield. This is a good feature in the only map you can get spawn killed (Training Arena team deathmatch), but I don't get the reason it's in all the others. If you are fast enough, you can use those seconds of invulnerability to clean capture points or just dealing damage without getting any back (which is garbage). It's mostly abused by scouts, assassins and few fast tanks. It shouldn't be there where it's not needed.
The game models. As a gunner I can tell you that the models in air have some big problems, throught experience I've got the idea that the hitbox models don't really keep up with the fast flying movements. I'll explain why using the head hitbox.
In case of just a normal jump standing still, the speed is low and you can still hit the head without any problems, but you can already notice something. The hitbox is not on the head, it's placed slighlty lower (shoulders). Now you may think it's not a big deal but in reality it is, because it means that the faster you go more delay there'll be with the hitbox model.
It even happened with scout that I could actually hit under the shoes and get hit under that. (With bomber's Q we have some other issues, like no head hitbox at all or just weird effects of the skill).
I'll add more later.