"Monetize" lectures or questions, say $1 per lecture?
If you really want to encourage people to post useful questions, create a market for them! Once a lecture has been purchased, it could be used as-is, or be edited modified by the purchaser to suit their own needs. The copyright notice would always include the original author, but would change to also include the new author as changes are made.
Not only could this allow people looking for questions to see which have been purchased most often (providing a more credible measure of popularity), but it would also provide question authors with funding to offset their student fees and/or generate some income... which would provide them with an incentive to drive traffic to your site. This idea is probably premature at this stage... but might be an attractive one to plan for down the road.
I think a low price per lecture would encourage people to re-use others' work more readily, and provide the useful measure of popularity mentioned above. Questions/lectures that have not had any student fees associated with them for 1 year, or which have not been purchased in, say, 1 year, could be dropped from the database as a "self-cleaning" mechanism. As long as authors can "export" their work to their machine (and perhaps re-import it later), this should not cause undue frustration from teachers who offer a course every few years.
Please don't do this.