Stress
Baseball is a multi-discipline sport requiring a lot of different skills, as well as a sport with a lot of different specialty positions. That can get confusing sometimes. Seriously, you pretty much have to be able to run fast (unless you're a catcher), hit hard (unless you're a pitcher), and throw the ball across half a mile of green field into a glove the size of a shot glass (okay, everybody has to do that).
The things you need to focus on will depend on your position…but you're probably going to focus on all of them anyway. You never know when you have to step in for a pitcher who got hit by a snowplow or an outfielder who's trapped under something heavy in his dorm room. That means a lot of cross-training as well as more specific training in hitting, pitching, and fielding. Be ready to run, too: healthy lungs mean you won't collapse like an emphysema patient while running down a deep fly ball.
You'll need to work very hard for long hours in order to keep your mad skills at their maddest. And you need to juggle that with the same class schedule and studying requirements that all the nerdy kids have. Like we said, you really need to love baseball. She is a harsh mistress, and if you let her down, she's going to drop you like a funky bassline.