The biggest thing I had to get used to was the delete key only goes backwards and there is no forward delete. (Well, there is a way, but no key on the keyboard to do that. It's annoying for the first 2 or 3 days.
As Borkya mentioned, the other big issue is whether stuff you had created on your Microsoft machine would be usable on your Mac. Mac has an "office" suite called iWork for Mac. It includes the three big "office" type applications (MS OFfice equivalents in parens): Pages (Word), Keynote (Powerpoint), and Numbers (Excel). Unlike MS Office w can only read and use itself, the Mac applications CAN open documents, spreadsheets, and power points created in Word, Powerpoint and Excel. SOme functionality is lost, but they can be opened and used. Also, the three Mac applications (Pages, Keynote, and Numbers) CAN export stuff you create in them to MS Office formats (Word, PPT, Excel) for use on Windows machines. Moreover, all three Mac applications have a function where, say, you get an email from a colleague in Word format (ubiquitous in China), you can open it in Pages, edit it, and then click a menu item to send the result back to the original emailer in Word format so the luddite can use it in Word. In short, the Mac office apps have full import and export functionality with MS Office. MS Office has neither functionality. Plus the Pages, Keynote and Numbers stuff looks so much nicer and works so much easier just for creating content.
You can even test it out, now, because Apple has just released online versions of all three FOR FREE usable from any machine, even PCs, although the on computer versions work faster and are a little more robust.
http://beta.icloud.comThe hugest benefit I've found from switching to Mac is that everything works together and the machine knows where everything is. Documents, photos, video, music, podcasts, etc. It's all accessible within each of the programs. If I'm building a Keynote (Ie PPT), and I want to add a video or music or photo, no need to fire up all those different apps, the Keynote app can read their content and show a popup box with the various content and just pop it in. It keeps track of where everything is rather than you trying to remember whichever dozens of third party apps you added and where they stored stuff.
Plus, Mac semi-automatically updates all apps (free) whenever something changes. No worries on out-of-date or even "pirate" versions creeping onto your machine. And did I mention that you almost never have to worry about various viruses and stuff no matter how many students stick their USB in your machines port to copy a lesson.
There's a lot of stuff not in the basic Mac manual that will amaze you with what you can do with them. I stumbled across an English copy of "Mac OSX - The Hidden Manual" written by Richard Pogue of the NY Times in a Chinese bookstore once, shortly after I got my first Mac. I spent that Spring Festival reading through it and got completely hooked. I also found 2 copies of it in my university's library and a few of my students who had made the move checked those two volumes out and passed them around.
There are lots of websites out there with good advice on using Macs. Since you're new, you'll probably want to scroll back through the older articles, but they'll teach you a lot. Once you get re-oriented, you'll find your workflow is much easier and more organized and efficient, and wonder why it took you so long to switch.
http://mac.tutsplus.comhttp://www.imore.comAlso, spend some time in the help menu of the various applications and watch/read some of the Apple Help stuff.
Welcome to the bright side.