This release is amazing!
Widgets are simply web pages that can exist outside of the browsers UI, they use all the technology already present in the HTML renderer, added with the PNG alpha transparancy without formal UI window chrome. This is not really another "feature" in the sense that it was always built-in, but it has opened the possibility for some really neat features. http://my.opera.com/community/dev/widgets/
Bittorrent is treated as another protocol like HTTP and FTP, as it should be. Obviously this will not replace a fully-featured client, but for general use it is great.
For power users, Opera's user CSS handling has been improved by allowing a directory for storage of all your CSS files. Remixing doesn't get easier than this!
A nice feature taken from Firefox is the "add search", basically this scrapes search fields on web pages, and a new UI gives you full control over your search engines.
Tab thumbnails are now available, and you can enable them also for ctrl+tab switching using opera:config.
A small, but neat feature is the source viewer (now syntax-highlighting), allows editing of pages, including blank pages so you can code in HTML and let it preview!
Of course, the big changes are under the surface. Merlin, Opera's next-generation rendering engine is really amazing. It supports a huge range of standards with a very high level of compliance. And amazingly as always, the same rendering engine can be run in mobile phones, which proves the efficiency of its coding.