Agree with the above post regarding the intangibles.
As far as the player himself, a lot depends upon the position. Size and arm strength are always important, but each position has specific skills which should be above average (ie speed for outfield, lateral movement for middle infield, solid glove for 1st base, etc).
It also depends upon the age of the player. A younger player needs to have future potential and solid grades to advance to the college field.
All this said, there are always players who surprise everyone and perform beyond their apparant ability. That brings us right back to those intangibles!