Note that if you are installing Alfresco 4.2a manually instead of with the binary installer, you'll need JDK 1.7, Tomcat 7, and if you are running MySQL, MySQL 5.5. The binary installer ships with LibreOffice 3.5, so if you want to match the dependencies in your manual install, you might want to make the switch from OpenOffice.
The new rich media gallery view will be installed by default in both the binary installer and WAR distribution. But the new Google Docs integration is only installed by default for those using the binary installer. If you are running a manual install you'll need to also download the Google Docs AMPs.
Please hit this release as hard as you can and provide us with feedback. If you find a bug, please confirm it on an out-of-the-box install, search Jira to be sure it hasn't already been reported, and then write a bug report with as much detail as you can possibly provide.