How do you deal with bad sentences/mistakes in the source data? Even the best dictionaries I look at often have very odd example sentences (at least in Japanese/English dictionaries). Do you have any plans to vet for things like this?
You measure word difficulty by frequency, but do you do any heuristics for sentence difficulty?
Do you have any idea if you method works? I'm not attacking you, what your site does is very similar to what I did on my own to learn a second language, but having hard data would be great.
Again, I really love the site, keep up the good work!