Learn your new x words and crowd sourcing.
I like the 'Learn new x words' section that is displayed at the end of each lesson where we are invited to enter our own sentences with the newly learnt words.
Nevertheless, I still don't get why we have to cram our sentences is this so tiny text area and why we don't have options to improve the formatting (by the way same comment for the 'Questions' section).
In this section I usually end up writing sentences I'm not sure of (is it grammatically correct? Do they make sense? ...) and I would like them to be corrected.
It led me to think that there may be an aspect of the duolingo's community that has not been explored yet. The fact that many users are are not English native speakers but Spanish, French or German native speakers or are fluent in these languages (it's a personal impression, could be interesting to have some statistics here!).
So why not using this diversity to correct each other sentences (e.g I correct French sentences and native German speakers correct mine). Everyone would benefit from it!