It seems like Apple disabled the "Press Backspace to go back a page" feature in Safari 6 due to users having complained about losing text entered in text fields when they accidentally pressed the backspace key. If you liked this feature, you can enable it again using this command:
defaults write com.apple.Safari NSUserKeyEquivalents -dict-add Back "\U232b"
If you ever want to disable it, there are two possibilities. The first, if you don't have any other shortcuts for Safari, is to run this command:
The second method is a bit more time-consuming. You need to first create a keyboard shortcut for Safari, for the "Back" command, in the preferences. Then, if you apply the first command above, you'll be able to delete it. If you don't have a shortcut set up for the Back command before running the first command above, it won't display in the Keyboard Shortcut preferences, and you won't be able to delete it from the preference pane. It seems like using the defaults command to set up a shortcut only displays that shortcut in the preference pane if you already have a shortcut created.
Applications such as TextEdit and the iWork apps have the new iCloud storage window. This window displays when you open the applications, or when you choose Open from one of the apps. If you click on the iCloud button, it shows a linen-background panel; if you click on On My Mac, you see a standard Open dialog.
If you have a lot of files on iCloud, you may want to make folders to group them. This isn't obvious from the panel, but if you are familiar with iOS, the trick is easy. Just drag one file on top of another to create a folder, just as you do with apps on iOS. Double-click on the name to change it. You can move files into and out of folders, and when there are no files left in a folder, it disappears.
In the new Reminders app, you can select several reminder lists by clicking on one, then Command-clicking on others. When you do this, the reminders will all display in one window, with a header saying, for example, 3 Lists. A smaller header will show the name of each list, with each reminder under the header for its list. If you then click the Hide Reminders button at the bottom-left of the app, you get a very neat reminders app that shows all non-completed tasks in a single list.
I found that the RSS Visualizer screensaver still works if you reinstall it from a copy of Lion.
In a Lion backup, if you still have one, go to:
/System/Library/Screen Savers/RSS Visualizer.qtz
Copy it to:
on your Mountain Lion Mac.
[kirkmc adds: I haven't tried this, but there have been other submissions about copying screen savers from Lion to use in Mountain Lion. If you're missing your favorite system screen saver, and have a Lion backup, it's worth checking to see if it will work in Mountain Lion. Personally, I'm still using the Basic Black screensaver.]
If you leave certain applications running without any open windows, OS X will quit the application when it goes to the background. This happens with some Apple apps such as TextEdit and Preview, and perhaps others. There is no user setting to change this in preferences. Here is a Terminal command line to change this feature:
[kirkmc adds: What's interesting is when OS X kills an application using this feature, the app still shows up in Activity Monitor. Matt Neuberg has a good overview of this at TidBITS. This has been around since Lion, and I'm surprised there hasn't been a hint yet. The submission specified Mountain Lion, and it's possible that this "feature" is more aggressive in 10.8, but I found mention of this command from before Mountain Lion.]
Arrow keys now allow you to move through events in Calendar. If you select one event, you can move around through all your events by using the arrow keys. Note that you can't highlight an all-day event using the arrow keys except in Month view.
Another behavior that might be new: if you hold down the Control key, a left or right arrow key will move a highlighted event right or left across days and the up/down keys will move the event in 15-minute increments when you are in Day or Week view, or move it up or down the calendar in Month view.
[kirkmc adds: Is this new in 10.8? I don't use iCal; um, Calendar.]
In Safari 6, with the new combined URL and search field, if you type search terms into the field, then press return, you'll get a Google page with the results, but you won't get a search URL in the address field. Some people want to save searches at times. Macworld's Dan Frakes tweeted an AppleScript which does just that and Many Tricks developer Peter Maurer improved on it. Run the script and it will get the URL for the frontmost tab then send it to the clipboard, from which you can paste it where you want.
tell application "Safari"
set the clipboard to URL of current tab of window 1 as string
As Dan Frakes later pointed out on Twitter, you can also drag the favicon to a finder window. You can double-click the resulting file to redo the search, or you can press the Spacebar to Quick Look it and see a live search with clickable links. For a search, the favicon is the magnifying glass icon at the left of the search field.
So, how many other ways can you find to save a Google search URL?
Update: Jordan Kay posted an even shorter version on Twitter:
tell application "Safari" to set the clipboard to URL of current tab of window 1 as string
I saw this early on in Mountain Lion, but forgot to add it as a hint (and I'm surprised no one has submitted it yet). One of the interesting features in 10.8 is the ability to rename files from the title bar. To do this, hover your cursor over a title bar, then, when the small downward-pointing arrow displays to the right of the name, click on it and choose Rename... The name in the title bar will become highlighted, and you can rename it.
(H/t OS X Daily whose post reminded me to post this hint.)
There is a gesture you can use on a trackpad to view Notification Center: swipe with two fingers from the right edge of your trackpad to the left. This works on a laptop, but with a Magic Trackpad, this really isn't easy to do, since your fingers can't slide from a surface next to the trackpad. All it generally does for me is move my trackpad to the left. (And if you happen to use the Magic Trackpad with your left hand, I'm not sure what would happen.)
So what I did is set up a hot corner on my desktop Mac, the one where I use the Magic Trackpad. To do this, go to System Preferences, then Mission Control. Click on the Hot Corners button, then choose Notification Center from one of the menus; the one for the corner you want to use. So I have it set to the top-right corner, which is logical, as that is where Notification Center lives. I just move my cursor to that corner and the Notification Center sidebar displays. To hide it, I just click elsewhere, or move the cursor to the same corner again.
If you have multiple conversations visible in Messages, you can cycle through them using a keyboard shortcut: press Control-Tab to cycle down, and Shift-Control-Tab to cycle up.
There seem to be some inconsistencies. When testing this, if I had my cursor in the text field, sometimes the keyboard shortcut wouldn't work and would instead highlight the name in the To section above the conversation.