I’ve attached my (unfortunately long) letter below, if anyone else would be willing to take a look. I’m hoping it was/will be received well, in spite of the fact that I’m a returning student … I’ve removed some of the class jargon, but otherwise the note’s as-is. (And thanks, again, MaineLonghorn!) 
"Mr. X:
I’d like to thank you for making [class] such an enjoyable class. I’ve learned so much and consider myself lucky to have you as a teacher.
When I first joined [class], I was intimidated by the course. I’m constantly realizing how much I have to learn (and, at the time, I thought that was a bad thing!). I can’t tell you how many times I felt like admitting defeat, like declaring myself a horrible speaker/writer/communicator.
While I can’t say I don’t still feel that way, you’ve changed my opinion of learning. It’s exciting to take risks, to test the limits of my skills. To a large extent, this is due to your reminders that writing is a long-term art. You helped me believe in myself, when I felt I was at my lowest. You gave me confidence in more ways than one.
Though I value your encouragement, I’m also thankful for your honest feedback. I remember a comment you made in September: “[Essay] is really repetitive – so fix it!” Your willingness to isolate seemingly inconsequential faults and techniques is much appreciated. “Sassiness” comes to mind – it pains me to say this, but I should definitely consider it. [This is a reference to a class joke, regarding my sometimes “cold, clinical” prose.]
On another note, you make me feel supported when you say, “Please feel free to let me know if any questions or concerns arise” – and genuinely mean it. I’m especially grateful for all your support back in December. That meant and means a lot, and I’m glad I could finish off the school year. [I had run into tuition problems.]
Perhaps most importantly, the eagerness you express to co-learn with us makes me feel like a partner in my learning experience, when it’s so easy to feel like a “receptacle to be filled.”
[Quote from class reading.]
I know you have a lot to share, but I love how you articulate how much I have to share, as well.
As an educator, your title can ensure a role of heightening my knowledge in academics. But you also help me see my uniqueness and strengths. I admire your efforts to get to know us – however startling it is to be “understood.” (And I’ll never forget the Buster Keaton .giffs. They’re just too good.) [Another class reference.]
All of which is to say: thank you. Thank you for being genuine. Thank you for the inspiration you provide, for the enthusiasm that makes every class enjoyable. Thank you for being one of the few great teachers out there. May you inspire others to achieve the greatness you have.
Sincerely,
[My name.]
P.S. My apologies for emailing so late into the summer. I was waiting for final grades to come out and missed any notifications!"