(Just an idea) Marking weak words
In the vocab section you can see your words' "strenght". Duolingo has a system of 4 gold bars, which show how well did you memorise the concrete word, according to how many times was the word seen. So, it should work like if we have 4 gold bars next to some word, we know it perfectly. But there are words which are just too dificult for my brain. Sometimes when I learned too much at once I feel like I do not have enough brain capacity for the last words. So the result is that I don't know these words enough. They need more practise, but they are full golden so they won't appear in my practise for a long time.
So, I think that there should be an option of marking words, which I feel that aren't yet "settled" in my brain, though I saw them just a short time ago. These words should appear in the refreshing with the non-fully gold words. For the end, I'm sorry for my not perfect English, but I hope you understand what I was trying to say :). So, what do you think?