[AutotelicBrown]

I'm assuming you mean only the demo? I'll answer assuming that. Hope my limited knowledge on HCI can be helpful.

First page showing available topics is pretty similar to other MOOC platforms, it looks nice and for the most part it works well. Not much to say about that.

First thing that I get clicking the link is a massive contrast change, and the sudden change in how the page looks forces me to look around to get my bearings. I'm no graphic designer but the overly gray design doesn't feel good to the eyes to me.

About the tree itself, I feel the fields are too verbose and occupying too much space for me to grasp the whole structure without having to think. Adding up to the fact you'd realistically have to deal with taller trees with more branches, a more compact look seems warranted (e.g. putting the descriptions of each node as on hover information or omitting it altogether until it is clicked). More on that point, I legitimately took a while to notice I could click on other nodes because of the lack of feedback and not being obvious that a node is selected in the first place.

The right column describing the contents of a node seems good to me, although the "Complete" button functionality isn't clear at all (is it only tracking locally my progress, am I submitting something somewhere or what) and I can't rollback if I press it.

Additionally, the page completely breaks if I try to browse with a smaller window. I hope it works with internet explorer.

On another note the tree idea in itself is pretty interesting, I can easily see it being useful with a proper implementation.