Those are ones I think are the worst: the ones that are closed minded.
Everyone comes in with diffirent backgrounds, diffirent natural ability, diffirent body types, or past injuries. It's the willingness to learn that makes someone a good student, just as it's the willingness to teach that makes a good teacher. Some people come in and are so uncoordinated that they can't shuffle forward without leaving half their body stuck to the mat. What's beautiful, to me at least, is when these people keep coming, and keep getting better. For them, something as simple as shuffling forward and back, is improvment.
Others come in with past training, and can pull off beautiful throws that they learned ten years ago. However, when they're taught new techniques, they question so much that they end up just using their old body mechanics again and again. Somewhere along the way these people lost the beginers mindset and until they regain it, they'll never progress.
"Seek not to follow in the footsteps of the men of old; seek what they sought."