The big news is that the Opera 10.5 pre-alpha offers users a taste of Opera's new JavaScript engine, known as Carakan. The next generation of browser wars will be all about JavaScript speed. Carakan for Windows is seven times faster at JavaScript than the Futhark engine used in Opera 10.10.
CSS3 and HTML5 Support Enhanced
Although speed is clearly the big focus with the 10.5 pre-alpha, the Opera Desktop team has also added support for CSS3 transitions and transforms, as well as more HTML5 support for stuff like persistent storage.
Opera's CSS3 support is actually more advanced that what is currently being used in Safari 4 and in Google's Chrome (I haven't tested it against the WebKit nightlies). Namely, rounded corners with border-radius and the variations in box shadows are far more complex. Even CSS3 Transforms and Transitions -- something that has been part of Google Chrome and Safari 4 for quite some time -- is more advanced in Opera 10.5. You can check out all the new features here.