The biggest issue that I see pop up with unfamiliar Mac users is they're used to Windows' assumption that if no documents are open, the program should quit. Mac OS assumes that if you wanted to quit the application, you actually would have quit it, not just closed the document.
The end result is that people don't pay attention to the little white dots at the bottom of the dock (which indicate a program is running) and end up running a dozen programs, wondering why their computer is so slow. The bottom line is to Command-Q that crap if you aren't planning on opening up a new doc soon and keep an eye on those little white glows in the dock.
Pretty much all your familiar shortcuts are there, they just use the Command key in place of the Ctrl key. Here are a few other handy ones:
Command-Space: Brings up Spotlight. Some people use this to launch pretty much whatever app or document they want.
Command-Shift-3: Captures the screen and saves it on the desktop.
Command-Shift-4: Brings up a selector to capture only part of the screen.
Option-click outside the foreground app to quickly hide it. I find it's the best way to get something out of my way fast.
Command-Option-Esc: For force-quitting apps. Basically the Mac's Ctrl-Alt-Del.
Control-scrollwheel (not sure how this works with a trackpad): Zooms the display for fine pixel work.
Double tap (not click) on trackpad: Zooms in on a web page element in Safari, like in iOS. May work elsewhere.
In the Finder, use columns view almost all the time (it's #3).
Learn the trackpad gestures. They're so awesome, I'm thinking of getting a trackpad for my desktop.
For apps:
Unarchiver for an elegant way to extract almost any archive.
Pixelmator for an image editor that will make you hate Photoshop for all but it's most powerful features.
BBEdit if you need a top-quality text editor.
Keynote for presentations.
Transmission for a BitTorrent client.
I hear gushing praise for Fantastical, though I don't care enough about calendaring to use it myself.
"The worst part about these reviews is they are [subjective]--and their scores often depend on how drunk you got the media at a Street Fighter event." — Mona Hamilton, Capcom Senior VP of Marketing
*Image indefinitely borrowed from BrainBoxLtd without his consent.
