What is teaching?

So, every other Saturday I have classes in Katowice, a neighboring city. I love these Saturdays, despite the fact that it means getting up early (which I'm no longer used to), walking into the center of Sosnowiec to catch a bus, which gets me to Katowice ridiculously early and so on. But the classes I have there are fantastic. The students are interested, talkative, engaged. I never have to tell them to focus on the lesson, most of them need little encouragement to speak, they're smart, they ask questions, it's sheer joy. And because of all these things, it's easy to have fun. Because I don't have to spend my time quieting people, or repeating myself because they weren't listening, we can get through the exercises and really use English. Which leads me to wonder about some of my other classes and my responsibility as a teacher. What is teaching, what makes a good teacher, and how much is the teacher responsible for any given classroom environment? I would really appreciate comments on this post, any stories or thoughts people have about teaching, teachers they've had, good and bad, etc.
I can't help but contrast these wonderful classes to some others I have, where it's a struggle to get a response of any kind. I ask a question, dead silence. I ask a direct question to a specific student, I hear "I don't know." What I wonder in these moments, is how much responsibility I bear for this reticence. Some classes, the students are clamoring for airtime, in others, I'm talking to myself for 80 minutes. What can I do to, how much is it possible for me to do, to bring these quieter classes out? Or is it just that people are different, so classes are all different, and it has little to do with me at all?
Lord knows, I've had some bad teachers in my time. I give first prize to my high school math teacher, Mr. Ames. He'd tell us stories about bow-hunting deer in between doing examples on the board with little explanation. Based on limited empirical evidence, math and science teachers seem generally unable to comprehend why their subjects aren't self-evident to their students. Most of the ones I've had tend to quick explanations followed by a certain childlike disbelief and petulance that their students didn't get it all in one go. Mr. Ames was the worst of this breed I'd ever had the misfortune to encounter. He had his own proof in his head that he just couldn't shake: math is logical, I understand math, therefore humans understand math, thus and so my students, being humans, must also understand. And I didn't. Ok, I also didn't care, but nothing in his presentation of the material encouraged me to care either. I suspect he also believed that the relevance of the material was also self-evident, and thus didn't require explanation.
Contrast this with, say, one of the greatest teachers I've ever had, Ruben Van Kempen, known familiarly as VK, who every year teaches with infinite patience and enthusiasm hundreds of teenagers the basics and more of drama and the theater, from costuming to stage management, acting and directing, set design and building, dramaturgy, tap, singing, etc., winning awards as well as hearts and minds. I date the onset of my adult personality to my first year acting course with him, as I learned, by being someone else, to express myself and be comfortable in my own skin. In my high school, the football team sucked, and the drama department was cool. Roosevelt High School in Seattle, and the Washington State Theater competition changed the musical part from competition to showcase because we won every year, due to the efforts and inspiration of VK. Certainly it was he who brought such infectious passion to those who would otherwise never have cared about theater, though some of us caught the fever earlier (myself, I must credit my mother here, summers with Shakespeare in Ashland, and numerous local performances that she always made time and money for, and thank you for that mom).
The point being that good teachers can reach and inspire many students, and bad teachers can turn off and discourage many, but some students are unreachable or would persevere regardless, so how does one decide if you're any good or not? I hate to measure my performance by tests, I teach English, a language, a means of communication. I honestly don't care if my students can use the future perfect continuous properly; I want them to be able to speak, to transmit ideas, and some of the best at that have terrible grammar. But they can express complex ideas that are comprehensible and that's great. What do I do though, with the ones who express nothing? If I measure my success as a teacher in that fashion, have I failed with these students? Or do these students honestly have nothing to say?
So, any questions, any comments, responses or stories are welcome. I believe teaching, education, learning and studying are the foundations of civilization, so please contribute any thoughts you have.