Long time user and donator to your beautiful cause and presently a 38 year old college math major student. Biggest issue I have is that the screen in regular view is too small (and I play this stuff on a big tv) and the screen in full view gets obscured at the top and bottom by titles and menus anytime you move your mouse or press pause, which I often do to take notes. I'm sure others have issues with this too as many of my fellow students take notes from these lectures. Also the captions bug is super annoying. I think I have an easy fix to both to offer.
When you go to a course, select a topic, then select a topic section, you're taken to a screen that isolates that section. When you scroll to the bottom you can find a link to view the video on youtube. And when you do this you can choose "theater mode" which is absolutely perfect. It's near full screen and doesn't have any obscuring when you move your mouse around. Making these links more accessible from the main course page instead of taking such a long path would really improve the user experience.
Otherwise I love everything as always and thank you in advance for my coming Bachelors in Applies Mathematics!!! I owe you more than I could ever repay.