Assuming that the point of comparison is Windows (since it’s a rough XP/7 analogue), any difference in behaviors, patterns, or conventions that might differ from what a long time Windows user would expect, including things that some might write off as insignificant. In particular, anything relating to the user’s muscle memory (such as key shortcuts, menu item positions, etc) needs to match.

The DE needs to be as close to a drop-in replacement as possible while remaining legally distinct. The less the user needs to relearn the better.

For example, practically every text box in practically every Linux system handles ctrl+backspace by deleting a word. This clashes with a Windows user's expectation that ctrl+backspace deletes a word in some system applications while inserting a small square character in others.