One feature I would really welcome is to be able to flag a kanji in some fashion during Reviews so that I can find it easily later.  For example, as I am reviewing I often encounter stories that aren't working for me or that I just don't like; and I would really like to be able to quickly click a little icon or something somewhere so that I can go back later, find that story easily and revise it.  This is distinct from my list of failed kanji (not everything I fail to remember requires revision...sometimes the kanji just hasn't made it into my long-term memory yet).  When I look at the list of failed kanji (or the list of kanji for which I have stories), I would like to be able to easily see the ones I have decided I need to do some more story work on...maybe they will be listed in a different colour, or maybe have a little flag icon next to them, or something.