I've always found that you can draw a box with four squares. On the top are "can explain material to moms" "can't explain material to moms". On the side are "clear communicator" and "unclear communicator".

The intersection of "can explain to moms" and "clear communicator" is what makes a good teacher for me.

In school some TA's could explain to moms but were unclear communicators so they didn't make good teachers.

In my work there are fantastic communicators in the sales team, but their grasp of the subject matter is such that they throw around lots of industry vocabulary but wouldn't be able to explain it to moms. This makes them bad teachers.

Great teachers/professors always had both great communication and a mastery of the subject. From my time at MIT the perfect example of this type of teacher/professors was Walter Lewin for 8.01 classical physics.

I highly recommend watching his lectures at http://ocw.mit.edu/OcwWeb/Physics/8-01Physics-IFall1999/Cour... (the picture of him on the website says it all.)

edit: grammar.