There are some aspects of PrE 12 (and some earlier versions), that differ by platform. Steve Grisetti has published a list of those differences: http://muvipix.com/phpBB3/viewtopic.php?f=103&t=12342
Most of these differences are with Effects that are available in the PC version of the program, but missing from the Mac version.
ATR has also listed some of those platform differences in his Blog posts:
Please take a look at my blog post on the use of the Difference Matte as a possible replacement for the keying effects not present in Premiere Elements Mac. This blog post would apply to Premiere Elements 11 as well as 12.
As for other Premiere Elements Windows and Mac differences, I have reported some in another of my blog posts directed to PE11: Mac Information Round Up