As people decide what study course to take up, it might be helpful to pick a course that is not very popular. Why you ask? Well, if the supply is higher than the demand, then the chances of ever finding a job in that field reduces. (For those planning to get into employment that is.) Anyhu, there are many other factors that play a role in choosing a study course, and you can read all those here, here, here and here. You can compare the change in popularity of study courses among students since WS 09/10 here.
For some of these courses, their popularity is just as high as the drop out rate. (Read: Less than 50% of University students in Germany graduate)
Below are the favourite courses according to the registration in last winter.