Japanese professional baseball stresses fundamentals to a far greater degree than today's MLB. How often do you hear about a young player in the U.S. coming up and being described as "raw"...or described as "still needing to work on the basics?" That doesn't happen in the Japan leagues.
For both pitchers and hitters, power is not stressed as much as skill. This is why you seldom see an overpowering player from Japan coming to the U.S....but at the same time the players who do come here tend to do all the "little things" well...baserunning, hitting to the opposite field, bunting, etc.
Finally, Japanese players are expected to play every day from the time they are in middle school...The reason you see many players, especially pitchers, break down once they are here is because many of them have literally played thousands of innings.
For example "Dice-K" Matsuzaka became a legend in Japan while still in high school. During the national high school championships (which are as big as the Final Four in Japan), Dice-K led his team to victory by pitching over 30 innings in three days...including a 10 or 11 inning no hitter. That would be unheard of here...but is simply part of the Japanese mind-set.
A body only has a certain number of innings in it, so while Japanese players tend to have "mature" games, they also have "older" bodies by the time they get here.