According to the official list of changes, this build is a beta release of Opera and, at the moment, it is powered by WebKit rendering engine. However, as development progresses, the source code will rely on Chromium and Blink engine.
The old functionality and looks have been preserved to some extent, but there are plenty of modifications. One of the most significant is separating the mail component, which is no longer built in, but available for download individually for Windows and Mac.
Another major change is the new tab page, which now has triple functionality. Apart from the old Speed Dial, now it also offers access to bookmarks (Stash) and a news feed with top stories from around the world (you can customize both country and language as well as the news category).
Speed Dial’s functionality has been improved by allowing you to group the items and now features a search function. The address bar (called Smartbox) allows predictive results that can be filtered by the search engine.
Also new is the Off-Road mode, a feature that uses the server-side compression technology available in Opera Mobile.
Keep in mind that this is a preview release, and new features, functions, and improvements will be added selectively and iteratively.