This post by Panic Engineer Steven is an excellent, not too techie, description of security issues facing OS X and Apple’s answer for it in 10.8: Gatekeeper.
“Although security is a vital feature for Apple, developers, and users alike, being unable to run unsigned code cuts a lot of really great things off at the knees. You wouldn’t, for example, be able to just download and run an open source project unless it had been submitted to and reviewed by the App Store. Highly disruptive software (think Napster or BitTorrent) may have not been able to exist on the Mac platform since it would have been likely to run afoul of Apple’s App Store guidelines. Major vendors such as Adobe and Microsoft might have withdrawn their support for the platform, being unwilling to cede 30% of their revenue to App Store distribution.
So, for a while, there was a great deal of consternation among Mac developers, including this author, that this might be the route Apple would take. In recent years, Apple has shown a trend of following the most hardline possible stance that will benefit users and Apple, often at the expense of developer freedom, and gradually backing in certain affordances (push notifications, for example) as user-impacting problems became evident. So it seemed feasible that we’d wake up one day and Apple would decree that all Mac apps must be sold through the App Store.
But instead, Apple went to considerable effort and expense to find a middle ground.”