I deal with a lot of PDF documents and until now was using Lion. I used to Quick Look PDFs all the time, and pinching used to make it full screen. But even in full screen, because most document have a portrait orientation, many times the text wasn't readable enough. Now, pinching in Quick Look, both normally and when full screened, zooms in or out like in Preview. I've tested this only with PDFs. I tried with a JPG, and pinching did nothing to it.
[kirkmc adds: Yes, I get this to work with PDFs, but not with other graphic formats. ]
If you haven't noticed, Web Sharing has been removed from Mountain Lion. According to Apple, if you need Web Sharing, you should probably look at OS X Server.
Nevertheless, many people want to use Web Sharing, notably to run a local server to test win site design. I've come across two tutorials. One by Brett Terpstra and another by Neil Gee. Finally, Tyler Hall has created a Web Sharing preference pane
Personally, I don't need Web Sharing generally, but I have used it in the past to test web applications. So if you need it, check out the above links.
To play your Mac's audio over an AirPlay device (like an Apple TV or AirPort Express), the standard way is to open System Preferences, then click on the Sound pane, then the Output tab, then select the device.
There's a much quicker way, however. If you have the Volume menu visible in the menubar, clicking on this icon displays a simple volume slider. But if you hold the Option key when you click on the icon, you can select audio inputs and outputs directly, including nearby devices compatible with AirPlay.
[kirkmc adds: This isn't new, at least the part about using an Option-click to choose input and output devices. It's worth pointing it out again, however, because you can now choose AirPlay devices as well from this menu. As noted in the comments below, it seems that this menu only displays the last selected AirPlay device.]
Previously, 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 view it in Quick Look. In Mountain Lion, this no longer works. You can, however, click on the graphic in the Messages window, then press the space bar to view the image in Quick Look.
You no longer need to right-click and display a dialog to be able to edit the name of a bookmark in Safari' 6's Bookmarks Bar. Just click and hold the bookmark; the name will become highlighted and you will be able to rename it in the Bookmarks Bar.
As a commenter pointed out, this is a feature of Safari 6. I edited the hint before Safari 6 was released, so wasn't aware that this would be the case.
In Lion Safari, you'd see an RSS button whenever you landed on a web site that had an RSS feed. This was a great way to quickly subscribe to that feed: just click the button and your newsreader would open and you could add the feed as a subscription.
Safari in Mountain Lion does not have that button, but developer Daniel Jalkut of Red Sweater Software has created a Subscribe to Feed Safari Extension. As Jalkut describes it, this extension "adds a handy button to the toolbar that, when a page offers RSS or Atom feeds, can be clicked to easily open the feed:// link, which should automatically open your favorite news reader."
Jalkut says that it's a beta, but it seems to work just fine in my tests.
You can easily pause the Notification Center in one click. Press the Option key while clicking on the Notification icon in the right end of the menu bar. This will pause the display of notifications until the next day. To reactivate Notification Center, you can Option-click the same icon again, or you can display notifications at the right of the screen by clicking on the Notification Center icon, then toggle the Show Alerts and Banners switch from Off to On.