Notes only offers three default fonts, and there is no GUI option to change these or add others. You can, however, add fonts by editing a plist file.
Open /Applications/Notes.app/Contents/Resources/en.lproj/DefaultFonts.plist (or the equivalent for the language you use), and you'll see three sets of text like this:
Copy one of them, paste it at the below those three sections, and add your preferred font and size. Note that you can specify the weight with "-Light," "-Bold," etc. Save the file, relaunch Notes, and choose your new default font from Format > Font > Default Font.
Many of the "main" folders of OS X can be accessed with shortcuts in an Open/Save dialog. If you don't know these, they are Command-D for the Desktop, and Command-Shift-H for your home folder.
With the advent of iCloud, there is now a new shortcut to save files in that location, for apps that support iCloud: Command-Shift-I.
PDF files with horizontal lines and rectangles, but not created as PDF form files, can be filled in as if they were actual PDF form files. Clicking on a line starts a text field the same width as the line. Clicking on a small rectangle toggles a check mark. Rectangles large enough for text entry are not treated as check boxes, but as text fields the same width as the box. After entering text on a line or in a text box, the text may be moved anywhere on the document and retain the formatting. And unlike PDF form files, text attributes as well as text field widths can be changed at will.
[kirkmc adds: I made a test document with Pages, and it works as described. This is pretty neat. I don't know if this is new in Mountain Lion, or if it existed before.]
Safari lets you choose from three search engines: Google, Yahoo! and Bing. You may want to use a different search engine, and Matt Swain's AnySearch Safari plugin lets you do that. It can also turn off Safari's 6's "omnibar" so it doesn't search, and only functions as an address bar.
As a bonus, his KeySearch extension can expand the use of the omnibar to let you search on many sites, such as Wikipedia, Amazon, IMDb, and any site you wish to search, just by using keywords. For example, you can type "wiki Marcel Proust" to search for the Wikipedia page of that author.
Just go to All My Files and the files will show up in their respective categories (or organized another way if you prefer). The path bar for these files lists their location as simply "iCloud." If you don't have All My Files visible in the Finder sidebar, you can add it by going to the Finder's preferences and checking it.
[kirkmc adds: This is interesting, but it's too bad there's no "iCloud" category to find them more easily; they're mixed in with all your other documents. Of course, the whole point of iCloud is that you don't worry about where the documents are…]
Well I, and many others, thought that this was missing, since there's no longer an arrow to click on in the search field, make that the "omnibar." But it turns out that there's an easy way to get to your search history. Just click in the omnibar, or press Command-L, and press the spacebar. The Recent Searches menu will display, along with the options to change search engines.
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
end tell
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
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.
After installing Mountain Lion, I was looking for an easy way to convert the e-mails I receive into reminders to act on later. After trying in vain in Mail (right clicks, menus, etc.) I found the solution is simple.
Just drag an e-mail from Mail to Reminders and a new Reminder is automatically created with the subject of the e-mail as title and a link to the e-mail in the notes.
[kirkmc adds: To be honest, I expected there to be a clearer way to do this: a toolbar button in Mail, or a contextual menu item. I think a lot of people - like me - use their inbox as a to-do list. Being able to add them to the Reminders app can be very useful.]
