I just noticed today the obvious feature "sort by amount of times failed" when looking at the detailed list of kanji.

It would be really nice if there was a review mode where the site presented the kanji in that order- those that you have failed most on first, and then gradually towards those that you fail less. It would be a great way of not only pinpointing but also reviewing and hopefully start nailing those kanji that you are most prone to fail to remember.

Come to think of it, the review could be programmed to present kanji in all sorts of orders. This could be just one.