Microsoft used to be all about preserving backwards compatibility, even resorting to crazy hacks in Windows 95 to make sure games like Sim City would run despite bugs in the game. But that Microsoft doesn't exist anymore.
It started with VB.net not being compatible with VB6, and continued with other products too. Microsoft obviously doesn't care about making sure IE10 works with old web apps targeted to IE6, Vista broke the old crappy XP apps that didn't understand non-admin users, and Office often has problems getting the formatting right for documents created in older versions. Those are the obvious examples, but there are many more smaller and more obscure ones.
http://www.joelonsoftware.com/articles/APIWar.html
Not saying I disagree with all of the decisions - e.g. the old insecure XP apps were broken anyway. But Sim City was also undeniably broken, and yet the old Microsoft took ownership of the issue and fixed it anyway.
And in Apple's defence, they did bring out Rosetta when they started making Intel Macs, and supported it for about 5 or 6 years. Microsoft did no such thing with Windows RT.