Windows is a pain to program for and use largely because it's filled with assumptions and code that date back to the eighties. It is very difficult to innovate with the boat anchor of the past shackled to your ankle.

Think how much easier it would be for Microsoft to improve security if, for example, they could remove the assumption that it's okay for apps to blithely write stuff to the \Windows directory. Well, Mac OS X has done exactly that. Most of its system directories are tightly locked down.

In fact, Apple has completely chucked their entire operating system from the eighties and started over. The result is a much more modern OS that is far more pleasant to use. Yes, there was a real fear there that, since people had to chuck OS9, they might move to something else entirely, but that didn't happen, for the most part.

Think about it this way. Which customers would you rather have: the people who are willing to buy new computers and software every few years, or the guy who is stubbornly holding on to his DBASE app from 1984? For me, the choice is obvious.

* 'Classic' support enabled Mac OS 9 apps to run on OS X for many years.

* Rosetta allowed for apps written for a completely different processor architecture to run on Intel (until 10.7, aparently).

Apple also made these moves before the bulk of 3rd-party applications were running natively on the new systems/architectures. The fact that Apple has been able to change low-level OS & processor architectures over the past 10 years is quite a feat.

Product quality tends to degrade (sometimes significantly) because more often than not it's necessary to compromise design, which in turn complicates code and causes more bugs. It has an engineering cost because it takes longer to develop the product. And perhaps most importantly, it can severely restrict how aggressively you can push your platform forward in each new version.

Having the freedom to move OS X (and iOS) forward aggressively is an important reason why Apple focuses its business model on selling products to consumers. Consumers tend to be looking for the latest and greatest, and each individual consumer makes her own buying decision. On the other hand, if an IT manager can't keep his VB6 internal clunkware running on your new OS, he's not going to buy 10,000 seats.

That said:

Microsoft's backwards-compatibility policies are great for businesses, who naturally have many special considerations when it comes to legacy applications.

Apple's policies, on the other hand, seem to be more geared towards improving the experience for the user. They deprecate libraries and functions of the OS that are no longer relevant to how computing gets done (or how they want it to get done) on the Mac. This helps to ensure that applications keep up with the times.

Microsoft don't have this philosophy at all, and so Windows 7 often feels like an arbitrary mash-up of the last 25 years of PC operating systems. There are many many applications in W7, for example, where buttons and interface elements still have a Windows 95-ish feel to them, because they haven't been updated to the new system themes. A famous example in Vista (corrected now in W7) was the 'Add Font' dialogue, which had been exactly the same since Windows 3.1. Another example is mIRC, where (if i remember right) some decades-old Unicode work-around forces the application to use non-Aero title bars.

At the risk of sounding like a snob, the whole experience -- to a consumer who appreciates good user interfaces -- just feels kind of amateur-ish, really.

I'm not sure where Apple got this reputation for poor backward compatibility. They've jumped platforms multiple times and have gone to heroic lengths to preserve backward compatibility as well as cross-platform and cross-architecture compatibility.

That said, the removal of Rosetta support seems uncharacteristically premature for Apple.

Sure, any ARM NetBook or tablet. But they're fixing that.

Combine that with requiring CS5, and Lion will be a very expensive upgrade for many people.

Other OS vendors can carry decades of cruft and baggage for reasons of compatibility with older applications, and that really slows down development, and the testing costs scale upward for whatever you can't retire. You end up implementing the Windows XP box in Windows 7, the pain that has been IE6 replacements and upgrades, and various other forms of API and run-time and source-code compatibility, and translators.

Some of this zombie software lives on ten and twenty years past expectations, and longer.

Apple throws features and hardware under the bus when it gets in the way of their skating to where they think the puck will be.

On the other hand, 10.5 runs fine on ppc. As a poster above noted, your old OS does not magically stop working after release of new version.

You could be worried about developers creating intel-only apps, which you won't be able to run, but that already happens on a big scale because almost nobody uses PPC macs anymore. This has nothing to do with Lion.

The bigger problem with Lion may be the dropping of 32-bit macs. Most people don't really know if their mac is 64-bit (Core 2 and up) or 32-bit only (Intel Core's, which is mostly the early Mac Mini's, iMacs and Macbooks I think). It's good for developers who want to write 64-bit only apps though, because once Lion has been adopted and SL support is dropped, they can just say 'Requires Lion', instead of the somewhat awkward 'Requires Snow Leopard on a 64-bit Mac' right now.