After watching some non-technical people browse the web, I think all modern browsers are pretty much equivalent (as far as the user is concerned). Normal users don't install extensions, they don't care about easy bookmark access or browser speed (as long as the difference isn't in the order of magnitudes) or really any of the things that browsers might do differently. It's just slightly different window dressing around websites. So I think cleaning up the Opera user interface as the article mentions, while nice, won't make a difference to user numbers.
When products are effectively the same, marketing is the ONLY thing that matters (bundling aside).
Now having said all that, there is of course a big difference amongst web browser when it comes to power users and as a long time Opera user, I find that it enables me to browse the web in ways that are not possible in Firefox and Chrome.
1. Good defaults (in terms of functionality and settings). I can install Opera on a new computer, connect to Opera Link to import bookmarks / searches, change three settings and I have my complete web browsing experience set up on another computer. I don't have to install ANY extensions, widgets or what have you.
2. The reason that I don't need extension is partially because some functionality is already included in the browser, and partially because the address bar in conjunction with bookmark aliases replaces a ton of extensions I might need to have and acts in many ways like a command line to the web. Bookmark aliases let you specify some "command" that you type in in the address bar that invokes a bookmark. So I can type "hn" into the address bar, hit enter and it will take me to hacker news. In addition to that, the address bar consumes javascript. That means that you can set aliases for bookmarklets and for multi argument quick searches. So if I type "bm" (for bookmark) into the address bar, it will open the window to bookmark something on delicious. The same thing that would happen if you click on a delicious bookmarklet button... only you don't have to click on a button. And it will get synced when you set up Opera Link on a new install. And you can hook up any old snippet of javascript you can think of to such a command.
3. Find on page acts as an element selector. If you see a link on a page that you want to click on, you can do "Ctrl + F -> type in part of link name -> enter" to "click" on the link. I know that FF has extensions (vimperator?) to do something similar, but again... that's one less extension you need.
In short, with a very small combination of extra features, Opera will save you a million mouse clicks with zero setup / configuration hassle.
(I once wrote a blog post about multi argument address bar searches: http://www.pushingbits.net/posts/google-date-range-address-b...)