The reason why most old Macs don't support Mountain Lion is because of the integration of the newer version of OpenGL. The graphics chips that are used in those Macs have been long gone in terms of upgraded drivers, and rewriting them wasn't worth it as clearly some/majority of us had those 2006 models that were on their last legs anyway. I'd rather them worry about security issues on older systems than writing needless driver updates anyway, IMO. The new feature in ML was mainly iCloud, and I have no problem just using the website on my iMac G4 running Tiger and it's easy enough to get the server information to use with the Calendar, e-mail, and the other apps that support CalDAV.
My theory about the iPad 3 being replaced really quickly (besides to update the port for Lightning) is because of issues with the A5X chip overheating.
The original iPad could have clearly supported most features in iOS 6, but then again like I said in my earlier post, it was missing features that are standard in today's iPad. If you're using the iPad as a reader and a web browser, it's still perfectly fine for that, but if you're using it for recording and processing music, or even editing a picture, there are some APIs that wouldn't work very well with it, especially when it has to deal with the system UI in the background.