Suggestion: "heart jar" to ensure solid understanding
So I figured I'd learned enough German vocabulary for one day, but wanted to preview the next skill.
I tried out the skill, without particularly trying to get it right - it was just a preview. Well, with my nonexistent efforts I somehow finished the skill with no hearts left. And then the lesson turned gold - I had completed it and could move on, even though I barely knew a single new word from the lesson. (I retook the lesson, of course.)
Then it dawned on me: this could be happening to other users. They take a lesson, and when they hear the trumpets they think they know everything and move on. Then they get halfway through the tree and only then do they realize they don't know a single word. (I have seen this happen to users in the past.)
My suggestion is a little complicated. At the very least, Duolingo wouldn't count a lesson as mastered unless finished with at least 2 hearts remaining.
But a better (and more fair) way would be to display the skill's strength, and the lesson doesn't count as "completed" until the lesson's strength has reached 100%.
I was thinking that each time you take the lesson, your remaining hearts could be placed into the lesson's strength jar; when the jar is full, you move on. The jar could hold 3 hearts, so if you finish with 3 hearts on your first take, you can move on immediately. Otherwise, you have to keep taking the lesson over and over again until the hearts from all those attempts have added up to 3. That way, you can't move on until you really understand the material, and you'll be less likely to break down halfway through the tree.
I personally never had any troubles with this. If I didn't learn the material the first time then I reviewed it when it stopped being gold. I would hate this idea just because most of the time when I lost a heart it was because of something not quite relating to the language, such as a valid answer that wasn't accepted, a typo, or even just that I pressed the submit answer button instead of the keyboard in the app. If the fact that something like this happened even once means that I have to take the lesson over again, I probably would have been severely demoralized from the start and quit before even getting to the first keyhole on the tree.
This is just my opinion, but this technique would not work at all for me.
I like the idea because it is fairly close to what I do myself (I never go onto a new lesson unless I get full hearts on a review), but I can see how it might frustrate others. Duolingo is good at letting people pick their own learning styles, and I think you can do this for yourself (just remember how many hearts you finished a lesson with and repeat it until you've filled your imaginary jar), without Duolingo having to enforce it for everybody.
Maybe the jar could have more hearts, maybe. Or the hearts in the jar could disappear over time and you can't move in in other lessons until you go back and fill up the jar again. Very good idea!!!
I started on Duolingo quite some time ago, and then left off for maybe a year. Now that I'm back, I'm missing a feature I used to like very much. Duolingo used to display how many hearts you had left for each lesson in the same spot as they have the four-segment gold bar now.
I like to keep on re-doing the lesson until I can finish with all the hearts left, but now, unless I make a note of it somewhere, I don't remember if I've finished off the last lesson completely when I come back the next day. Perhaps I haven't been back long enough to see anything "decay", so I may not completely understand how the system is supposed to work now, but I liked he idea of showing "hearts left" that I remember.