I did not join the discussion so far (I rarely post here), but I'll chip in and say that character acting is the epitome of good animation. It is painfully hard to get right - which is why more often than not, especially in anime, character acting relies on symbolic action and visual cliches to convey a character's personality /emotions etc- and even harder to do well. And this goes beyond technicalities like physically believable movement or hiding your key frames. When you do character acting you have to defeat an obstacle that action or effects animation does not have: the incredible familiarity the viewer has with the subject. It's easier to fake a couple of swords slashing or a bomb blowing up than a smile or a frown, because your audience sees one a lot more often than the other and can spot the flaws in it much easier (we could bring in a side-subject about the uncanny valley and all that at this point).
"Weight and momentum" is not all that matters. There's also expressivity, what a cut of animation "communicates" to the audience.
Sadly, most "sakuga" fans are not actually fans of animation at large, but fans of good action scenes. Just looking through this thread makes that a dead give-away. Or looking at those MAD abominations that are 90% 'xction and 'splosions. Or maybe I am the odd one. Whatever.