If you're coming from Windows:
Tips for new Mac users:
The green button at the top of a window does not expand the window to fill your screen. Very few windows need the full screen, so what that does is expand the window so it's large enough to handle the content. This will always leave some of the background visible.
The red button does not always quit an application. It often merely closes the active window. The exceptions are those applications that cannot have two or more open windows. The "light" by the application's icon in the Dock will indicate if the app is still running.
You cannot cut and paste files in the Finder as you can in Windows Explorer. To move or copy a file from one place to another, the Mac way is to open both the Target and Source folders, then drag and drop the file or folder. There are nuances depending on where the Target folder is. For example, hold down Option while dragging a file to copy it to a folder on the same volume.
You cannot see the size of a file or folder in Icon View by holding your mouse over it. You must highlight the item and use Get Info. Or you can use List View, which will show you the size. There are at least four ways to access Get Info, three of them involving the mouse.
You can't make a Finder Window display folders first in all circumstances. There are simply some file types that the OS sorts ahead of folders. There are work-arounds. The simplest, but perhaps most inconvenient, is to put a space at the beginning of each folder name, and sorting by Name.
If you have two folders with the same name, but have a few differences between the contents of same, don't move one to the enclosing folder of the other. OSX will not automatically merge the contents of the two folders. Instead, it will overwrite the target folder with the one you're moving. The differences in the target folder will be lost forever. You will be prompted to confirm that you want to overwrite. Be sure you know what that means before saying yes!
You really don't need anti-virus software. It will just slow your computer down, and there isn't a single virus out there that will infect a Mac. If you run Windows on your Mac, you do have to take precautions for that. And, you can pass a virus on to a Windows user. Just be smart about where you go online and you'll be fine. It's also not necessary to run a defrag program. The Mac OS takes care of that for you. Some Mac users still do it, myself included, but you'll be fine without it.
You cannot selectively delete items that you have "stored" in the Trash. Sorry.
Now, if anyone knows that some of the above will no longer be true in Lion, please enlighten me!
