A correction was made to this story. See below for details.
The majority of the changes in this Opera alpha release are aimed at developers. The average user will rarely, if ever, come into contact with them. That doesn't mean they're not important, though, with further support for CSS3. These changes include sourcing fonts, transparency rendering, animation framerates, and two key evolutions in Opera Dragonfly. You can use the DOM inspector to determine the source of traffic, useful for Ajax debugging, and the ability to edit attributes in real-time.
There are three minor but important user-level changes. It's hard to believe that HTML support in Opera Mail was missing before now, but you'll now be able to see all those holiday e-cards within Opera. There's also a spell-checker rolled in for text fields, not just e-mail.
The last new feature, automatic program updates, could be potentially risky. Certainly in Firefox it would likely lead to the disabling of many extensions, although there's not much of a chance of that happening in Opera with its smaller developer community. The bigger concern is one of control: do you choose which version of a program you get to use, or does the publisher of the program? As annoying as Apple's update monitor and nag screen are, they don't force users to update; you can opt out.
It took a little hunting to find, but the default setting in Opera 10 seems to be the more standard notification behavior. If you'd like to tweak your update settings, go to Tools, Preferences, choose the Advanced tab on the right and then Security from the list of options. The Opera update drop-down menu allows you to change the default to "Don't check for updates" or "Automatic updates".
The big news of the standards compliance will only take Opera so far if other browsers match that mark. It'll be interesting to see, as Google Chrome introduces extensibility, whether Opera will go that route or if it'll try to maintain its niche market as a solid and fast out-of-the-box browser. However, Opera 10 was surprisingly stable during a half-day of testing for an alpha release, crashing not even once.
Correction: This story initially gave the wrong name of the Java test I ran. It is called SunSpider.