Of those, I've seen spotlight needing re-indexing after an upgrade. But it's something so minor and easily fixed that I don't even consider it a fault of the OS

Of those, I've seen spotlight needing re-indexing after an upgrade. But it's something so minor and easily fixed that I don't even consider it a fault of the OS (actually, it could have as well been DESIGNED to do that, e.g to update the indexes to some new on-disk format). Silent DVD errors, install stalls, keychain and Mail accounts missing sound like genuine bugs. As for the rest, >- pathologically slow performance at first restart, usually on older RAM-constrained machines trying to re-thumbnail everything This doesn't sound like an OS bug, more like something to be expected. >- Now-missing JVMs causing significant problems during first boot Sure, but what uses Java at boot? The above and third party printer/scanner managers freaking out, sound like totally orthogonal issues to OS bugginess. If anything, I would _expect_ to have to install new third party drivers with an major OS update.