Gavin has taken care of most things worth saying.
There are two things I'd like to add, though. First... although it's often claimed that the BSD base is a big part of OS X's security... Apple has added a ton of things to BSD and it doesn't seem like BSD's vulnerabilities (on the rare occasion that they pop up) are anything but a small minority compared to the rest of the OS X vulnerabilities - OpenBSD, for example, has still had fewer vulnerabilities this decade than I can count on one hand (as far as I know) and Apple has beat that by a long shot. Discussion of Apple inheriting security from its BSD base should thus be taken *very* carefully. Second... looking at Adobe's vulnerabilities and market share, there is no good explanation (apart from advertising and previous popularity, which I'm not considering 'good') for the continued widespread use of their products - we already should have moved to something less painful than Flash a long time ago, and considering the many replacements for Reader (SumatraPDF on Windows, for example) it is sad to still see such high market shares.
Jeff