First and foremost an excellent teacher has all the qualities of an excellent student. She is articulate, open minded and friendly. She is excited about the subject and is able to communicate her amazement and understanding to her students by articulating her own intuitions and expertise in a way that is accessible to everyone. She also provides a bird's eye view of the subject by interjecting various tidbits from the history of the subject and pointing to stumbling blocks. You'd be surprised to see how many teachers act like answering a simple question is beneath them, speak in monotone, purposely ask trick questions to make the students feel stupid, dismiss questions as not worthy of an answer and just plain act rude and unfriendly.