I often forget to add a revision comment when saving a page, yet I cannot go back and add a comment to that revision, nor can I add a comment alone when editing a page (if I don't make any changes to the contents of the page).

It would be helpful to be able to add comments to particular revisions, and edit previous revision comments.

It would also be nice to be able to require a comment, unless marked "minor." In addition to this functionality, you could lump minor revision comments with major revision comments. This latter would require some sort of versioning system, similar to how SharePoint works, but would make for a useful page history system.