Here's another good idea that might turn out to be a bad idea once an obvious flaw is pointed out. I think I'd like to be able to easily see and print (a) the characters (or words) that have gone from 'not known' to 'known' in the last day/week, and (b) the ones that have gone from 'known' to 'not known' in the same period. seems to me these are the ones I could most usefully revise when I'm not on internet.
If it's done weekly, it could perhaps be included in the weekly progress report, to avoid having an extra button or feature.

Some of that can be done with the export tool; it can show you your least known characters.

http://www.skritter.com/vocab/export

Thanks, that's very useful.

I like the idea for forgotten words. The forgotten ones would be usefully studied by writing some sentences and getting my tutor to check them (I find if I use a word in a sentence or read it somewhere to give me more context it helps immensely for retention).