The ability to group open document tabs together into blocks manually a-la firefox in order to create workspaces set up for working on specific areas of a project without having to search through all the files and re-open just those specific related files each time you work on that area, nor if you have to work on two or more areas having all the files show up mixed together in a single huge list for navigating through.
Matt Wilson commented
This would be really helpful. It would also be great if the IDE understood which files are related and provided a means of grouping related tabs automatically (for example, an MVC view, controller and script files - using the MVC conventions).
This would also help avoid confusion when you have multiple views open with the same name (e.g. you wouldn't have multiple identical looking Details.cshtml tabs etc)