I'm thoroughly perplexed by the weapon masteries thing. I don't feel like it adds anything of value to the game, other than slowing down combat. It feels like complexity for the sake making the game feel more complex...for the sake of complexity. Sure, it adds more decisions, but not interesting decisions.
And it's also just so arbitrary. Sure, giant hammers might be good for knocking people down, but in most cases the mechanic associated with a given weapon could just as easily apply to almost any other weapon. (And, in fact, they will, since Fighters can arbitrarily assign any mastery to any weapon.)
Do you like it? Why?
I do think it slows down combat, but at the same time I don't think you can give martial classes more meaningful things to do in combat without slowing down combat.
However, I think Cleave, Flex, Graze, Nick, Push, and Slow are all great. I like all of those, and I don't think they slow down combat much at all.
Sap I like the effect of a lot, but I think it's mastery prerequisite of "no other abilities" is too strict. Simply put, weapons in that category really feel like natively bad choices, and Sap isn't enough to make up for it.
I'm moderately concerned about Vex. Advantage on every attack after the first against an enemy is really powerful. I am honestly surprised more people aren't similarly expressing concern about it, especially if Elven Accuracy remains an option. What especially bothers me is that it's effectively limited to Dex-based weapons. Dex doesn't need this kind of help. I think a better option would be that the creature can't take reactions until the end of your turn, or can't take opportunity attacks until the beginning of your next turn.
Topple seems like a ton of extra die rolling for those saves, and it feels like it'll be a little too obnoxious. It also doesn't care about the target's size or if it's flying. That's kinda weird, and probably is a problem.
The selections are basically arbitrary, but so is the damage type on spells. Why is it fireball instead of acid burst? Why is it lightning bolt and not flame lance? Why is it cone of cold and not dragon's breath? Just because. Saving throws are often similarly arbitrary. This one is Dex because AOE, but that one is Con because toxic gas. The game has to give options and make choices. Some of them are going to be arbitrary.
I'm still not sure about the Fighter ability to choose different abilities. It feels extremely narrow for what it offers, but it's also flexibility that nobody else has.
