What changes when you advance from one level to the next?
The only thing that changes is the number. It has no real meaning and doesn't do anything special.
Edit: It gives you some mostly useless Lingots too.:P
The only thing I can think of is how people view you; same people are going to trust the opinion of a person on level 23 than someone on level 8 in that particular language.
That's what I thought. I keep repeating lessons before moving on so I realty remember the lessons but I'm near the top of my tree. Some other people are on level 15 and finished their tree and others are on level 30 still halfway through the tree. I think looking at level to see your progress is better because you may only know a few lessons but you know them better than if you move on straight away.
Work at your own pace. Don't compare, higher levels don't mean you've learned all that's in each. Repeating is good.
This is something I've been wondering as well. I thought that the algorithm would start asking harder questions/sentences and accept less typo's etc. If that is not the case (as people suggest here) why not? I think it would make sense. Or is this too hard to build?