!. Strong ego- a devine belief in yourself that will persevere through any challenge, hardship, poor performance or adversity. Not to be confused with arrogant.
2a. A strong psyche- ability to think without being clouded by negative energy.
2b. Clutch- not affected negatively by external pressures.
3a. Knowledge of the game- how the pitcher will set him up, understanding of strategies, how to move runners/ where to hit the ball, opponents tendencies, etc.
3b. Team player- using knowledge of the game to his own and team’s benefit
4. Develops skills and moves to next levels of development quickly
5. Retainment of skills learned/ muscle memory- through repetition and practice
6. Desire to excel through refinement of skills/ practice
7. Impeccable fast-twitch muscle skills- superior reactions/ quick feet, hands, trunk
8 Footspeed- slow-twitch excellence
9. Incredibly accurate vision
10. Luck- The help of he right person and situation to rise from level to level from t-ball to the Hall of Fame

Note- size is NOT on this list. I played at 6’5, 230, and I really think it’s not essential. Also, I don’t think every major leaguer has all these qualities, But if I had to say which ones are most pre-eminant, I’d say
1.) fast-twitch muscle speed
2.) vision
3.) footspeed
4.) #4)-#6
5.) #1-#3b
6.) luck

Thanks.
References :
Former minor league player/ current HS coach/ umpire

The first thing is definately a strong work ethic, if you want to be any good. Ichiro is a perfect example. As a kid threw 50 pitches, took 50 ground balls, 50 fly balls, 200 live swings, and 300 pitches from a machine. It is also the reason most pitchers can’t hit. They simply don’t practice it. You lose what you learned very quickly in baseball.

The second I think is a little more tricky as I think there is a very fine line. Talent comes in handy, yet sometimes the more talent you have, the less of a work ethic you have.

Finally, it is nearly impossible to learn how to hit after the age of seven, I think. The hand eye coordination for hitting is simply too complicated.
References :

You need to know the sport;Be smart with the sport.

Not buckle under pressure;Its important that most of them don’t suck under pressure, because there are A LOT of pressure situations in Baseball.

Now of course you need talent in baseball.

Pitchers
-Excellent control with the ball; Need to be able to put the ball where you want in the strike zone.

-Trickery; Hiding the ball, throwing curveballs, making the batter THINK more and more to make HIM buckle under the pressure.
Also needs to be able not to over throw the ball.

-Arm Power; Arm needs to be strong; Pitcher needs to be able to throw fast, throw hard.

Batting:
Strength; Player needs to be strong, needs to be able to hit the ball hard and fast; needs to generate fast bat speed.

Mechanics; Needs to be able to have a smooth swing.

Fielding:

Fantastic Hand-eye coordination

PS-There is no requirement to where you are born that restricts you to play in the Major Leagues.

Anybody Can play!

A baseball player needs to have all of the above to play the game!
References :