I agree with you that on-line learners need good guidance to enable them to select suitable courses and make the most of their time, even though on-line courses are often very convenient and do not seem time consuming. It is easier at school when you follow curriculum and have set exam dates so that you know what to concentrate on, what to prepare for and at what speed you should progress at.
A good course description, course / curriculum map connecting the course to possible other courses and clearly spelling out the skills the learners should gain upon completion of the course should help. Indicating a level, for example if a course is suitable for Beginners in a given discipline or Advanced level learners, what are the per-requisites one needs in order to enroll and succeed on the course, also help to point on-line learners in right direction.
As Alan says – we often know about our knowledge gaps which leads our search for the courses but it may not be the case for younger learners, school leavers who are in limbo wondering what to do once they completed (or perhaps have not completed) their formal education.
This is such an exciting area with so many opportunities!!!