Here is the resumé of our contentions regarding grips:
Originally Posted by BoramicNYC
[G]rip is the single biggest factor in determining the characteristics of one's game.
Originally Posted by BevilDevil
But grip affects the typical swing path and racket tilt at impact. Sure, someone with an extreme grip can flatten out, and vice versa. But it's a matter of what feels comfortable and natural. And different grips have different ranges of what feels natural.
Originally Posted by Cheetah
[W]hy do 99% of players with western grips hit more spin than players with an eastern and why do players with eastern grips hit flatter that players with a western if the influence is very much exaggerated?
And, personally, I posted that grips were not this determining.
Regarding what BevilDevil posted on the first page:
I won't obstruct discussion with a detailed analysis unless required, but I'll answer each questions briefly.
For learning, I would advocate teaching macro elements first: provide them with a rough overall approximation of a great movement before asking them more nuance in their actions. I would teach the basic preparation, take back, swing and follow-through before telling kids about what their forearm need to do. It also answers your question regarding the learning curve: we build it in steps, adapting the simple structure by adding up new details every time.
For the amateur problem: can it be repeated? Well, the famous wrist movement is actually easier to replicate if you use a stretch-shortening cycle. It's that "natural pronation" people talk about on the forums. It simply requires that your forehand and hand are set in the right position as you swing forward and it's easier than actively trying to make it right every time.
Downsides? Well, if you close too much the racket face (like Nadal, most often), you also reduce the potential contact area -- it makes shanking likelier. Furthermore, on bad days, it's not impossible that you have troubles getting the hand position right which prevents the possibility of enjoying a super-humand pronation.