Let’s start with diff - If this is a text doc, diff seems easy. Show old and new text next to each other, somehow highlight new and old version. Immediate problem: what if you choose to highlight “new” with purple + underline, but user is already using purple + underline as a part of their text. Do you choose a different color? Or just hope it is “clear from context”?

Now for spreadsheets: the formula has changed, how do you show this? You can expand the cell to show formulas, but those can he huge, so your document can become unreadable. Or maybe just highlight the cell and let user click on it to see the formula difference - but then you can no longer see the changes at a glance.

What about things with no text representation, like conditional cell format? Are you going to come up with text representation just for display purposes, or are you going to design special text formatting dialog which shows “new” and “old” values?

There are hundreds of questions about GUI diffs which simply do not exist in texts.