Well-thought-out. That's the key.
I'm not going to go into detail. This isn't a technical or academic opinion. It's a very pedestrian opinion. Why do I like Opera? It comes down to a few features that I just fell in love with when I saw them.
Username and password completion
Opera frames username/password forms that it knows the information to in an unobtrusive brown-yellow colour. If you want to put in information that you know, go ahead and do it, it's not in your way. However, if you want Opera to fill it in for you, just press CTRL+ENTER, and Opera fills it in for you, and submits the form.
It's unobtrusive, it gives you the information you need to know via easy-to-understand visual cues that are not distracting, and it's much better than any other implementation I've seen in Safari when it comes to multiple known usernames and passwords.
In Safari, it auto-completes it for you while you type, akin to someone finishing your sentences for you. It's sweet, but not always fun.
All that empty space when you press CTRL+T for a new tab. The first implementation of tabs, inherited from Firefox and now implemented in all browsers, focuses your text cursor on the URL bar, so you type in things from there. Shortcuts even, can be put in through there through Firefox and Safari add-ons. Well, the guys at Opera are way above their game by putting in this feature called Speed Dial.
It uses the empty space of a new tab to show you 9 boxes with window previews that you can click on to navigate to. Each box will contain a cached thumbnail of a version of the website you visited. If you want to see what's the latest on them, just click refresh, at this window, and Opera will give you the latest thumbnails by reloading the page.
9 large buttons (taking up a lot of screen real-estate that was formerly unused), so your mouse movement need not be very fine (as opposed to navigating to the correct button on your bookmarks bar). You can use your mouse and keyboard in excellent synchronization. Left hand for CTRL+T, with the right hand still on the mouse, move quickly to a button, click, you're there.
These 2 features alone make me like Opera. The fact that it loads webpages fast, has fun visual indicators while loading pages, launches fast, and has, in my opinion, the best user-interface of any application I've used in Ubuntu Linux (running KDE) so far are still secondary concerns.
I'm sure there's more, but nothing worth me writing about. The guys at Opera have their heads in the right place from my analysis of the application so far.
I use Opera at work, where I work on Ubuntu Feisty Fawn. At home I still use Safari, because Safari is still more widely supported than Opera, and Opera's user-interface still isn't up to Mac standards.
I generally stay away from Firefox. A lumbering beast of an application, it takes for ever to launch, it's memory footprint is huge and it's really become persona non grata in my browsing experience.