The "enroll" button was a bit scary, I wasn't sure what that would do. I'd expose as much of the course materials as you can without forcing someone to enroll.
Didn't mean to scare you with that enroll button haha. Enrolling at this point really doesn't do anything other than give you full access to the course (you can then rate/review it, access the sections, and partake in the forums).
I have a story to think about letting people in w/out enrolling but am unsure how well that would work because then it is hard to gauge a) how popular is a course and b) should a user be able to review the course. Plus I intend to eventually allow course owners to charge for their courses if they so desire and want to get users used to enrolling since eventually that may have a checkout process attached to it.