First, if you previously used iChat to send and receive screen shots or graphics, you may have used Quick Look to view them. More specifically, if you had the iChat File Transfers window open, you could click on an inline graphic that someone sent you in a chat and press the space bar to preview it with OS X’s built-in file-viewer. In Mountain Lion’s Messages app, this no longer works. You can, however, click on the graphic right in the Messages window, then press the space bar to view the image in Quick Look.
Second, you no longer need to right-click and summon the Edit Name dialog to rename a bookmark in Safari’s Bookmarks Bar. Now, in Safari 6, you can just click and hold the bookmark; the name will become editable, so you can rename it then and there. This applies to Safari 6 as a whole, which is provided with Mountain Lion, but which is also available for Mac OS X 10.7.