1- I mostly use "student's enjoyment of the class" and I find it useful along with other activities but this one mostly keeps me in a full communication with my students plus it helps them in building self confidence in giving opinions and expressing themselves also I can notice the really hard working students through their feedback.
2- the end of the course exam is still the best way to check the students understanding , at least in my opinion , there are other ways like an opened class and an opened discussion with the students but mostly this is my way of evaluating students progress .
3- language can learned by repetition and that is why I use "recycling tasks" , if they can't accomplish a task in the first time , they can do it in the second time or the third , the purpose is to keep trying until they succeed and also they can learn from their mistakes.