First of all, I really like Duo, you made learning languages fun and easy!
I wish that the level system would be also based on knowledge, not only on work put in repeating different lessons. I mean- one user at lv.15 could have already completed his tree, while another user at lv.15 is in the middle of his own.
So here's an example of my suggestion- you can't get above lv.5 until your succesfully master your first checkpoint and in the same time, when you do master it, you automatically get to lv.5; there are more checkpoints and the end of the tree equals, say, lv.30, so everything above lv.30 would be based solely on consolidating your knowledge by repeating lessons.
Was denkt ihr darüber? :)
I think many people would not like that, too restrictive.
However, I do see your point, it would give levels more weight and a sense of certification. Basically when you would see the level, you know this user knows this up to this point and knows the material well.