All of the above, really. Correct understanding of the elements of a good grip: a strong fulcrum, the use of the fingers, the rotation of the wrists, the ability to control the rebound of a stroke; these things are very common in students who come to me specifically for "technique".
As others have said, posture is an issue, timekeeping - often coming down to not counting, any degree of knowledge with the established repertoire.
Most drummers that come through, self-taught or otherwise, can play along to music, but far fewer have a dynamic touch or a real responsiveness to what they're hearing.