From my own experience, and the experience of other Mac users that I know, I think that the quality of their software has had a marked decline in the last decade or so. The exact reasons aren't totally apparent, but it seems like there's too much rush to add new 'features' without fixing and refining existing ones, and that annual releases aren't giving them sufficient time to polish what's already there. It's no accident that Tiger was incredibly stable and 'complete' after having nearly three years of work done on it.

I'm not advocating they work for three years on each version of OS X, but even 18 months, and fewer new 'features' per release, would give them more time to improve the quality of what they're releasing.