I played D1 baseball for the past 3 years and am playing professionally right now.
I am a firm believer at being strong at contact--forearms, triceps, biceps. Forearms being the number one area to work on.
I would have to say that mechanics are the most important part of hitting a ball hard--without those-a hitter is doesnt have much of a chance.
"party like a rockstar, hammer like a pornstar, rake like an allstar."