Opera Turbo is the feature most prominently listed in promotional materials, so let's start there. Opera Turbo, turned on with the flick of a switch, will compress websites to maintain browsing speeds even in bandwith constrained situations like a slow WiFi connection or a choppy 3G connection. According to Opera's own tests, Opra Turbo can speed up the web up to about 8 times.
The interface has also been completely redesigned, and is now "streamlined and elegant" according to Opera. Tabs are on top, but in true Opera fashion, can be placed anywhere else too. Tabs can now also be displayed as website thumbnails, which could be useful on widescreen configurations.
A feature which should've been in there eons ago has now also finally arrived to Opera: automatic updates. No longer will an update require a re-installation, which should bring Opera up to par with the rest of the pack when it comes to up-to-date installations.
Web developers have also been taken into account with Opera 10. "Web developers can play with the newest update to Opera Dragonfly, our set of on-board, Web development tools. Opera Dragonfly, available in 36 languages, now allows you to edit the DOM and inspect HTTP headers. With a 100% score on the ACID3 standards compliance test, and support for CSS3 Webfonts, Opera 10 is ready whenever inspiration strikes."
You can get this latest release from the website.