Idea: 'scratchpad' to practise something I just had corrected
Firstly - hope you're sick of hearing how awesome duolingo is :)
Second - I think it would be really useful to have somewhere pop up that I could practise typing the correct answer (yes I know I could do this in notepad or whatever - but it wouldn't be as pretty as duolingo!) once corrected - would really help to build some muscle memory. I find particularly with French where its not always so clearly... phonetic... that even if I repeat the correct answer to myself several times I'm likely to screw it up next time I have to type it.