Masculine and feminine
I think that in Basics 1, Duolingo should teach students about masculine and feminine. I was fine because I've already had previous teachings in French, but a completely new learner would get very confused as to why 'le', 'la' and 'les' are all 'the'; and why 'un' and 'une' are both 'a'.
In the spanish tree they have some tips pop up next to the hearts that help in these situations. I hope they are planning it to do it for French too now that it's out of beta.