I'm currently working on extending it with word tracking functionality, so that you can see which words in an article are new to you, can track your progress through kanji by reading, etc.
Last time I learned a language (German), a small vocabulary was my perpetual weakness. Now I'm learning a different language (Spanish) with a different approach that makes heavy use of spaced repetition, and vocabulary is no longer my weakest point. Instead, listening comprehension is now my weakness. But with a decent vocabulary, I am now working on improving the listening comprehension using podcasts. If my vocabulary were weaker, this would be much more difficult as I would need access to a native speaker who would considerately restrict themselves to mostly use vocabulary I know.
For language learning, there’s one other wrinkle: you need to be working with material written in the target language, rather than written about the target language. Most of my language cards are Cloze deletions from novels I’ve read or museum captions, for instance.
See [0] for a review of the research on spaced repetition.