In my opinion, extensions for Opera are a really good thing. I've always felt that, done right, extensions would be an important feature for Opera users. Furthermore, I don't believe that the inclusion of extensions signals any change in Opera's guiding philosophy.
For one thing, when a user downloads Opera, he or she will still have immediate access to all of Opera's innovative and useful features. Opera is not a stripped down browser where normal users have very few features to boost their browsing experience, and only power users get the best toys (since they're the ones who are likely to look for and install extra stuff).
Opera continues to include cutting edge and legitimately useful features out-of-the-box, making it easy for new users to discover them--turning normal users into browsing heroes! As always, that's done while keeping the browser small, lightweight, and blazingly fast.
For those who want special functions, however, extensions are a great way to go, and they fit in nicely with Opera's existing means of customizing the Web experience, such as UserJS, Opera Widgets, Opera Unite, and changing the browser's look and feel.
I hope you'll give Opera 11 alpha a try! Visit http://www.opera.com/browser/next/ to download or learn more.