There have been several requests to make MediaViewer look like a popup which is on top of a page, instead of looking like an application that completely replaces the page. (This could be done by making the overlay transparent, or reducing its size so there is a border area where the underlying page is visible.)
The advantage of this is that the current lightbox design feels disconnected from the article; the user has no visual clues which would suggest they are still on Wikipedia. (Also, it has been suggested that the application-like UX gives users the wrong idea about how history navigation should behave.) The disadvantage is a more cluttered media viewing experience.