I've used every version of OSX including the beta. The last time I saw the kinds of bugs Leopard has appear in an initial release was 10.0. It has nothing to do with the number of people upgrading. It has to do with Apple getting behind because of shifting everyone to the iPhone and then rushing to not have to delay Leopard again. They should have bit the bullet and released it at MWSF.
I've used every version of OS X since the beta, generally upgrading immediately. I am also been bitten amazingly hard by .y releases in the past. Again, this is all anecdotal: many people seem to have no evidence of these bugs of which you speak, some do. How many cannot accurately be determined here.
This is also the most intricate and complex of their releases since 10.0. It has had the longest dev cycle, and has the most changes. It is not surprising that it has a higher raw number of bugs than previous releases.
Do you realize how horribly bad of a business decision it would have been to wait until MW? As someone else put it, people here seem to watch a bunch of commercials and declare themselves experts in marketing: Waiting the extra few months would have been a PR disaster in comparison (we'll have to wait to see if this hold's to be true, but it certainly seems so from the reviews thusfar).
As it is, the only place that I am even hearing a substantially negative opinion complete with cries for blood is... here. I see the odd negative review (mostly extremely positive with a handful of caveats like Siracusa's review, a few recommend waiting for 10.5.1, but in the aggregate positive).
Because it was the changes to the operating system that caused these programs not to work. Apple really should have worked with adobe to get patches out upon Leopard's release. It's really not in Apple's best interests to piss off the adobe professionals.
It is up to Adobe to keep their software updated, and if you depend on those apps it is up to you to ensure that they work before upgrading your operating system. I agree, it would be ideal if Apple upgrading the OS didn't cause bugs to appear in other pieces of software,
however Adobe's software is huge, massively complex, ancient, and still uses carbon.
As you say, it also caters to professional users. If you depend on the software for your livelihood you should be waiting to hear how it performs, not upgrading immediately every time there is a new OS release.