Oh, this again. I should put a website with this up...

I was the person who personally ran 10.6 security updates at Apple (10.6.1+), the "DRI". My team in the Updates Program office and I reviewed every single bug to determine if it should go in a security and stability update or wait for the next major version. Seriously, every morning we group triaged all Mac OS X bugs, both incoming and those nominated internally for us to look at and determine if it should go in an update. I packaged and audited the builds and tuned the delta vs full updates. I built the system that largely automated diffing "trains" for software updates (automastering).

The new version of the OS was always being developed in a branch/train, and fixes were backported to the current version as they were found. They weren't developed linearly / one after another. So, if you are comparing the most stable polished/fixed/stagnant last major version with the brand new 1.0 major version branch, the newer major is going to be buggier. That would be the case with every y.0 vs x.8. But if you are comparing major OS versions, Snow Leopard was different.

Snow Leopard's stated goal internally was reducing bugs and increasing quality. That is a fact, not marketing. I am not sure why people on the internet don't believe that, but I was there. If you wanted to ship a feature you had to get explicit approval from leadership and the bar was high. In normal feature releases it operated bottom up "here is what we are planning to ship" and in Snow Leopard it was top down "can we ship this?".

AFAIK Snow Leopard was the first release of this kind (the first release I worked on was Jaguar or Puma), and was a direct response to taking 8 software updates to stabilize 10.5 and the severity of the bugs found during that cycle and the resulting bad press. Leopard was a HUGE feature release and with it came tons of (bad) bugs.

The first .1 or .2 ALWAYS fixed critical bugs, because:

1. You had to GM / freeze the software to physically create the CDs/DVDs around a month before the release. Bugs found after this process required a repress (can't remember the phrase we used), which cost money and time and scrambled effort at the last minute and added risk. This means the bar was super high, and most "bad, but not can't use your computer bad" bugs were put in software updates...which was developed concurrently with the end of the main release (hence why .1 came out right away)

2. Testing was basically engineers, internal QA, some strategic partners like Adobe and MS, and the Apple Seed program (which was tiny). There was very little automated testing. Apple employees are not representative of the population and QA coverage is never very complete. And we sometimes held back features from seed releases when we were worried about leaks, so it wasn't even the complete OS that was being tested.

Software updates are always needed, though the issues they fix became less severe over time due to larger seeds (aka betas), recovery partitions, and better / more modern development practices. But I can tell you FOR A FACT that Snow Leopard had fewer major bugs over its lifetime, coalesced very quickly, and was extremely solid when Lion was released.