Since we have the option to show the description on the course page (MDL-27001), people start to use the description all over.
It's still good to reduce the importance it in the first step. (MDL-38614)
But another improvement would be to have a <! more > -tag like in WordPress. It stands for read more, and could be used here similar.
Teachers could write a description, that splits up into two parts: the short claim for the course page, and the full description ( including the first part) if the student has opened the activity. This does not bother inexperienced users, but reduce course build time a lot.
Right now this can be done in two time consuming steps with two element: by using 1) an additional text field on the course page and 2) the description.