Pearl's causality stuff and Jaynes' Bayes stuff are probably something a lot of LW readers want to know better. I don't know how ankifiable they are, I have a very poor grasp of either. My impression is that probability math is a bit more loaded with hard to formalize background concepts (cf. Tim Gowers' musings about just what random variables are) than some other fields. This might make a straightforward flashcard approach less useful.

Category theory and abstract algebra seem to start with a bunch of basic definitions which you could memorize with flashcards, but getting anywhere in them even with the definitions handily uploaded in your brain takes quite a bit of mathematical maturity. I tried to do CT with Anki, but got bored quickly since I didn't have any study plan beyond that and am nowhere near the mathematical skill where I could just start doing interesting stuff by myself using the definitions.