I would like a feature where multiple items can be selected, and tags added to all of them. When multiple items are selected, the right hand side might show all the tags they have and a count of how many items have each tag (or "all" if they all have it). New tags can be added in that pane to all selected items. This is primarily for adding tags, although removing tags might be accommodated in the same way.

Windows Live Gallery is an example of that very easy to use multiple-items-at-once tagging:
http://windows.microsoft.com/en-US/windows-vista/Using-tags-and-captions-in-Windows-Live-Photo-Gallery