There definitely are some different ideaologies about program design in switching from Windows to Mac (or vise versa) but general concepts are pretty much identical. If you're like me and started out on DOS (and still comfortable using the keyboard to do almost everything) you'll find it difficult to get around in MacOSX because menus simply do not activate when you hit the Alt (or Cmd) key. But of course most people started with graphical OSes and use the mouse more than I do, so it's an easy switch for them.
Just a few things I would suggest to keep in mind:
1) Mac's do support multi-button mice, they just don't ship with them. In my experience it's helpful to get one, otherwise you have to become proficient using a combination of the mouse and the keyboard to show context menus on desktop icons.
2) The PC's ALT key is the Mac's CMD ("open apple") key for the most part.
3) Most MacOS X programs do not fully exit when you close them, which allows them to "re-open" much faster. MacOS X, being a powerful UNIX system at its core, has GREAT memory management so it's okay to leave these active-but-not-visible programs running all day if you like.
4) If an app does lock up (it happened to me a few times in OSX 10.2 and only once or twice in 10.3) you can still click off that program window and then click onto the Apple menu at upper-left and select "Force Quit". This is very similar to Windows' Task Manager and allows you to kill a stuck program.
5) Check out sites such as
http://www.versiontracker.com and
http://osx.hyperjeff.net/Apps/types.php for lots of MacOS X freeware and shareware programs.
6) And yes, if you do have a friend or colleage who runs Mac and also has Windows experience, ask them to if they'd be willing to answer the questions you'll have. I cannot tell you how valuable that was for me in the first few weeks with my Macintosh.
Oh and 7) Keep in mind that this is not an either/or situation. You CAN have both a PC and a Macintosh. I run mind through a KVM (keyboard, video, mouse) switch so that I can run both side by side on my desktop with only one monitor, keyboard and mouse.