Well in general: Having less bad options.I’d revise to add this. Way too many tiny, pointless feats in 4E.
Its not only feats, there were also quite a lot of powers (a bit less for items), which were just really weak and did not really add much to the game. Of course its good to have options, but if there are too many which are not useful at all, it just makes it harder to find the ones which are useful.
Also to this part I think something 4E could learn from pathfinder is to have "Combat" and "non combat" feats. Some of the weak feats are more for flavour/non combat and just cant compete with the combat ones, but if you would get 1 "non combat feat" per lets say 5 levels, some of the weak feats could still have some value.
(Although I am not 100% sure about this, making the leveling up more complicated, is not necessarily good. In PF2 I feel like there are too many different feats you get and some of them are hardly remarkable).
Yeah I think you should only be able to know two reactions at a time: opportunity attack, plus one from your class. Having multiple off-turn options on one character is too much of a distraction IMO. And for interrupts, it should be only 1 per character.
I agree that reactions can slow down combat, and I am also not 100% sure if so many are needed, but they also make combat feel more interactive. They should, however, be simpler. Not disrupting the flow, needing an attack roll and damage roll. This will cost quite a bit of time.
Emberwind has "number of reactions per combat" as a stat, so if you want to do more reaction you have at least to invest in it. I think thats a good middle way.