I think it would be very helpful to be able to sort by user-defined tags.  While I know that I have the ability to filter by tags, this means that I can only view a portion of my collection at a time rather than view it all at once, sorted alphabetically by the tags that I have defined.

Obviously, this also implies that tags would now be an option when creating charts and printing reports.

If the reasoning behind this is unclear, here is my situation:
I have my DVDs organized alphabetically by genre on my shelves (Action/Adventure, Animation, Comedy, etc.).  This genres differ from the ones in the online profiles obviously.  When I want to print a full report of my collection, but organized as they are on my shelves, I have to filter each "shelf genre" and run the report for it separately.  This wastes both time and paper.  Also, I cannot see how many movies I have in each section without filtering each one at a time, because there is no option for it in the charts section.  Would love to see this option implemented to improve the ease of use.  Thanks!