Can I downgrade to Mavericks after installing Yosemite (beta/developer preview)?
To downgrade, boot into Internet Recovery with ⌥⌘R and select Reinstall OS X.



With regard to the original question for Yosemite Developer Previews, yes you can, but note that Yosemite DPs so far do seem to break most of the App Store. If you are still running an early Yosemite Developer Preview, you can downgrade using the Recovery HD (⌘R) as the Recovery HD was not upgraded. For later DPs, including GM and public beta, see above.