How similar is Microsoft Excel for Mac to Excel for Windows?
My Excel in the Mac doesn't open some password-protected files of my Windows ExceL: in Windows, the Excel passwords can have an almos limitless number of caracters, but in the Mac the password must be 16 or less.

The only solution I found was to reduce the number of characters in the password of the Windows files.

If you have Office for Mac 2011, there is a 100 percent compatibility ratio, as described on the BrightHub review.  

I work with both and personally, I find Excel for Windows much more intuitive and more user friendly, but basically you can do (almost) the same with both versions.

But the most important part is, the two versions are 100% compatible with each other.