The changes to the new tab are substantial, and represent the first major visual refresh to Chrome in some time. The new page splits up your Most Visited sites, Chrome Web apps, and Bookmarks into three distinct sections. There are arrows on the left and right side of the page to flip through them, as well as links at the bottom of the page. The browser saves the last one you were looking at, and returns to it when you restart the browser.
The redesign also includes an intuitive organizational feature. You can drag items from any of the three pages to the bottom bar and create custom categories for keeping them organized. Double-click where the new category label is blank, and a text box appears. You can also mix and match items in a custom category, so you can have your Gmail Offline Web app live next to your Gmail bookmark. To remove an item, drag it toward the bottom right corner of the page, and a trash can will appear for you to drop it into.
The full Chrome 15 beta changelog can be read here.