The refresh is rolling out to everyone over the next couple of days; Google Apps for Business customers that don't have "Enable pre-release features" box will see this update in a few weeks. If you have it on your account already, check out the in-product tour. If you'd rather have us explain what's new, read on!
As you can see above, Google added a number of filters, making it easier to narrow your search by type, visibility state, and other criteria; don't worry, sort orders like Name or Last Modified Date have not gone away. Priority Sorting (à la Priority Inbox from Gmail) is now an option in all views, allowing Google to push your most relevant files to the top of the list after considering a number of factors.
There's now also a preview panel on the right side of the document list that allows you to see a preview thumbnail, sharing settings, and other file exploring tweaks. If the file is a video, you can start playing it directly from the preview panel or from the video player. Similarly, photos can be viewed by clicking on the magnifying glass to open a full screen slide-show viewer.
A new Home view lets you choose the content you access the most often: right click a file and select "Don't show in home" or select "All items" to view all the files you've hidden. Collections, a combination of labels and folders, have replaced Folders in the left navigation bar. Much like with Gmail labels, collections allow files to live in multiple collections, and just like folders on your desktop, they can also be stored hierarchically.
Last but not least, there are a few general improvements. The interface has been streamlined. Keyboard shortcuts for selecting multiple files are now available: Shift or Control (Cmd on Macs) keys. Google also says the speed of the documents list has been improved thanks to an overhaul of the back-ends.