Keep grid display for cells formatted with a color fill
Something that has aggravated me longer than anything else:
Leave the gridline display for cells one formats with a color or pattern fill turned on:
For at least 30 years, the gridlines around the cell are turned off when the format is applied. One must replace them with actual cell borders which, I might mention, do NOT go away themselves if the color or pattern fill is removed. Oh, and select slightly the wrong color for the borders one is adding and it all looks subtly bad.
Simple enough to add a checkbox to the formatting tab to clear the gridlines. (Or to keep them: that approach would not affect existing spreadsheets since the default would be the faulty behavior of the past. And display would turn back on the moment the formatting was removed.