Here's a quote from that thread:
We haven't done the high school but my kids did try the math program through their school. My kids (DD8 and DD10) both hated the program for what I will call aesthetic reasons. The program was not visually interesting and the man who lectured had a dry, droning voice. You might ask if you could see a sample before you sign up.
EPGY sent me a sample of the geometry course materials. Aesthetics are definitely lacking. The course materials were LaTeXed/TeXed (so, all black and white).
Now, I wrote my M.Sc. and Ph.D. theses with LaTex and know how powerful it is and how far ahead of its time it was. But for me, LaTex isn't a good choice for course materials. Color is generally a superior way to highlight important points and is definitely better for diagrams. Actually, I livened up my thesis by including colored graphs. I just left big blank spaces on my LaTexed pages and then ran them through the color printer to add the graphs. I also glued color photos into the thesis.
Re: the boring man. I think that part of the Khan Academy's popularity stems from the guy's voice and the way he speaks. He has a nice voice and he changes his tone when he lectures, which makes him easier to follow.