How interesting
I find it very interesting all the talk of ditching legacy platforms. As a software developer, I see it as a tension, between legacy and moving the platform forwards.
MS chose to be backward compatible, this has rewards and costs, to my mind one of its biggest rewards for MS is lock in. Because a given version of software works across versions of the OS, the OS is perceived to have a longer life. The cost is that the OS has to continue to support features and API calls you would wish to deprecate. Or worse continue to support undocumented API because some whiz discovered just the right feature to make their software work (This is part of the reason you can use undocumented API calls in iOS apps).
Apple chose a different route they have a generation, if its outside that its gone. Its harsh, but it means there is less crud in the OS, in theory it is cleaner, and thus more stable (notice I say in theory).
On the other hand, I can run Leopard on old hardware, G4 hardware, and old CRT iMacs, that dont think you could do that with Vista.
So to my mind you pick your lock in, not forgetting that choosing an OS, due to hoe applications are sold means you have chosen your lock in. Personally, and I doubt this would stand up in court, if you have brought a license to use a given software then if you change platforms that license should still be valid.
As to Lion, well I dont know, I dont like the iOS features, the launcher or full window apps, i dont see the point. The resizing well thats nice, I have always considered it one of the UI things Apple has gotten wrong, but its no big improvement. The biggest downside for me, is the lack of a disc.
The versions things seems like a great idea, and nice to have it at the os level, I hope you can turn it off, on a system, directory, application and file level.
So while I wont be adapting it as soon as it is released I might in the future... I only adapted snow leopard this year.