Problem: The mobile web diff view features an enormous 'thank' button at the bottom of the screen. Sadly, I find myself reverting people far more often than I thank them. Quickly reverting a dodgy edit is the sort of quick editing that should work well on mobile. Unfortunately, the mobile interface makes this all but impossible without manually rewriting the text.
Who would benefit: Editors checking their watchlist on their phones.
Proposed solution: Provide an undo button adjacent to the thank button. Undoing an edit would work the same way as on the desktop site - open the editing interface with the offending text removed from the source.