On the judging impressiveness based on gameplay part:
The player with the most dominant looking gameplay doesn't tell as much about them as it does their opponents. That's the player who suddenly looks like a mess when put up against relatively stronger opposition. It all crumbles at that moment. Everything you thought was amazing about that player all of the sudden disappears as if they never were really that good, unless they actually are that good, which is a case of one in a hundred.
I've had many moments where I think a player is good because they looked really strong against relatively weak opponents.. But then look like absolute dogshit when they face a better opponent in the server. That's what should matter.
The games at the highest levels don't even tend to look impressive from any individual's PoV, because they tend to be evenly matched. They are constantly trading blows, it doesn't look dominant, therefore no one "looks" spectacular.
So from what I've seen, an important factor is not how they look, it's who they are playing and the importance of the game. How they look is 99% of the time a function of their opponents and not them. If they keep looking good after facing opponent after opponent, they are probably good. If they look insane because of the gameplay footage you've seen and crumble in groups, they are probably not good, no matter how you twist it.