My 2 cents:
I coached HS ball for the last 4 yrs and recently started working part time at a local baseball academy, helping out at clinics. Which, it goes without saying, qualifies me as an expert!!
Different ages groups for different clinics, ranging from 8-18. One thing I have seen WAY too much of, is a lack of understanding of how to:
a) hold a bat, and
b) swing a bat.
I think the biggest problem is kids not really watching baseball on TV and not playing 'sandlot' ball. I learned from the older kids in the neighborhood, and watching games on the tube. When we played outside, we would mimic the older kids and they would tell us when we were doing somethign wrong.
95% of the kids I see [including the kids on our HS team] are all arms, sweeping the bat across the plate, instead of driving it across; everybody stands 3 feet from the plate so they can hit an inside pitch, instead of keeping the hands back and driving the knob to the ball. Everybody has an aluminum bat swing.
Kids benefit from good instruction only if they apply it. Can't tell you the number of kids who have told me, when I suggested a slightly different approach to the grip/stance/stride "This is the way I hit" or "I've been hitting this way my whole life" [from a 9 YO!!! LOL].
Most kids don't want to put in the time/work to be a good hitter. [Nobody wants to put in the time to be legendary - greatness is not fast food!!] I ran into this at the HS - tried to institute drills, to no avail [e.g. hitting drills off a tee: "That's ***!! Why would we hit off a tee - that's for little kids" Of course that's why they stagnate, too, in terms of their development.]
I think too many kids are taking lessons, etc from too many people [how many kids you know see multiple instructors plus coaches?? or bounce from academy to academy until someone tells them what they want to hear?]. But I feel a receptive kid, receiving intelligent instruction will benefit, regardless of his age. Making instruction age appropriate is really the key.
Manny/Vlad, and everybody in pro ball [yeah, even Craig Counsel!!] are world calss athletes [regardless of what Krukker says]. Many of them start off doing weird things, but the vast majority, at the point of impact, are doing the right thing; even if they are not, they are successful b/c they have tremendous hand/eye coordination , unreal fast twitch muscles, etc. Joey Bagadonuts down the street will never be a pro player, because he's not a world class athlete, but also b/c he does things wrong - I think you kinda need one or the other. I think it's just kinda the way it is.
For 99% of us in the world, if we want to succeed at something, we have to do it the right way - I think that's what instruction is about, but making it appropriate for the audience - you can't teach biochemical engineering to 1st graders!! It's not a stretch to be able to show 8-10 YO kids that you hold the bat this way, stand this way, stride this way, and swing the bat like this. ANd then let their individualism fine tune it.
Long winded - sorry - gotta keep the windmills turning!