The problem: Very often it happens to me that I spend a lot of time selecting a shape with the polygonal lasso or lasso tool and only to realize after I completed the selection that I had pressed the wrong button for add or subtract selection, resulting in the loss of all the progress.
The solution: If there was a feature I would call "Reselect inverted", which undos the recent selection and applies the very same selection with the oppsite add/subtract mode, the time consuming work of re-drawing the selection was saved.
Although I know that there are much better ways to avoid the above issue (working with paths or in the masking mode), I can imagine that many other users are working in similar way like I am, facing the same problem in the process. Especially when using the very helpful quick selection tool and refining some details with the various selection tools, this problem can easily occur. For me it would a massive reduction of frustration if this feature was implemented.