The most notable difference in the GUI is the File menu.
On Windows each window of an application has the File menu at the top of that window. And closing the window Quits the application.
In Mac the File menu exists at the top of the screen all the time, it never moves. Its contents just change to reflect what application is in the foreground.
Closing a document's window does not Quit the application. Much nicer, I think, because you're not relaunching apps all the time. They can stay open even if you don't have any of that app's docs open.
The File menu always being at the top of the screen means that you can sling your mouse up there and click on what you want to do without having to look at it. In Windows, because the File menu is IN each window, you always have to look at if before you can zero in on it with the mouse.
Another thing I prefer about the Mac OS is how it handles removable media, CD's, DVD's, USB flash drives, external hard drives, etc.
When you connect any of those devices the icon for it shows up on the Desktop. No more going to My Computer to access a device that you just connected. When you're done with it simply Right-Click on it and choose Eject.