This move is substantial for Apple, as it's traditionally been quite strict about what beta software reaches the public - and here, we have an entire OS. Previously, developer accounts priced at $99/yr were required to access these special builds, so it's clear that Apple has reached the point where it believes more people testing = less bugs at launch.
Giving consumers access to beta OSes isn't a new idea - it's been de facto for Linux for as long as I can remember, and even Microsoft's been in on the action with recent Windows releases. For consumers, it means accessing the latest and greatest early - albeit with potential side-effects - and for companies like Apple, it means a more refined OS at launch.
While Apple fans haven't necessarily been plagued with bug-ridden OS X versions at launch, there have been some bugs that lingered through the private beta stage that have ended up driving people bonkers after launch. With Mavericks, a bizarre but simple bug with Mail made working with email a tedious task for some. With public betas, bugs like these, which are much easier to spot with more hands in the jar, should be ironed out in time for launch.
Nonetheless, users interested in taking pre-release OS X builds for a spin can sign-up for the free Beta Seed Program and download a small app which will cause the updater to begin looking for beta updates. If there's a "gotcha", it's that participation requires Mavericks, and presumably will always require the latest official release of OS X at that moment in time when a pre-release build becomes available.