What features from Lion will be lost when upgrading to Mountain Lion? I know Mountain Lion has some cool new features that Lion did not have, but will it overwrite any of Lion's functionality?
Of course.  In adding features, the behavior of previous features changes, not always for the better.

If you find features that were present in Lion but missing in Mountain Lion, please edit this answer to add your contributions to this list.


Mountain Lion drops support for some models of Macs supported by Lion.
Clicking the top right corner no longer triggers Spotlight (Spotlight's menubar icon is still present, adjacent to the new Notification Center icon).
The battery indicator in the menubar will no longer offer to show the remaining time.
X11.app is included, but that app alone no longer provides X11 functionality. When the user first runs X11.app – or an app that requires X11 – a dialogue guides the user to Apple's article HT5293 about XQuartz, which provides the functionality.
Mail.app and Safari.app drop support for RSS Feeds.