I'm also a big fan of the developer tools. While similar to Firebug etc, I prefer some of the minor differences in Opera.
I also think built-in support for torrent files is a really nice feature (don't know if any other browsers have caught up with this yet).
And the builtin RSS reader is better than anything else I've come across (not that I spent that much time looking ;-))
Chrome is a glorified data mining device and just feels overly sterile and corporate, FF grew complacent and has always been buggy, but Opera has everything I need built right into it:torrent client, note taker (very easy to use, super useful since I'l always taking notes on one tutorial or another), selective ad-blocking, the adress field doubles as a search field for any engine I want,it has the most customizable GUI and Opera Unite is awesome for file sharing- a couple friends and I use it to raid each others media collections every few months.
I can't stand when other browsers sometimes reload pages when I go back/forward, especially when I had text typed in forms.
Top feature that no other browser offers: fully customizable UI. Right click on any UI element of the browser and select Customize->Appearance. The Toolbars and Buttons tabs allow full control over the browser UI so I can move buttons and toolbars to my heart's content.
Fully customizable keyboard navigation. Customizable using the interface at Settings->Preferences->Advanced->Shortcuts->Keyboard setup->Edit. Make sure to check the "Enable Single Key Shortcuts" option on the "Keyboard setup" pane to get the most out of it. Check out http://www.opera.com/browser/tutorials/nomouse/
Fast history navigation: This one isnt actually enabled by default; set the following preference to 3 opera:config#UserPrefs|HistoryNavigationMode The history is fully cached so there is absolutely no delay between pressing the back button and viewing the previous page in history and it comes up exactly at the point you left it.
1) it is available on all the platforms I use (Linux, Maemo, Mac, Windows, Symbian) and syncs bookmarks to and fro
2) it can be secretly installed in my home on my work computer which runs a heavily customized old Debian without modern libraries.
* A lot of the features I care about have native support, meaning they look and perform better (visual tabs, mouse gestures, ad blocking, etc.)
* Great performance
* I think Opera is probably more innovative than anyone else in the space - many of the new features that have shown up in other browsers recently are copied from Opera. I've had speed dial for ages.
* On Windows, at least, I think it looks a lot nicer than any of the other browsers. Though I guess FF4 copied it a bit so maybe that advantage is less now.
Firefox is dead slow if you add all the needed functionality build in Opera and Chrome doesn't have much going for it (speed-wise check the benchmarks; and Opera is very responsive when I keep my 50+ tabs open, whereas Chrome cannot handle the load, freezes the computer and uses a huge amount of memory).
Oh, by the way, the Opera default is "restore previous session" and it does not close all your tabs when you close the browser so you are encouraged to have one continuous browsing experience; I've been keeping some tabs open for years.
Just a sneak peek.
I think all Opera's extra goodies are good only if you expect 75% of functionalities. E.g. Opera Chat still don't support encoding fallback (X-chat, IRSSI, mIRC, etc. does), or it is not very obvious to disable the built-in BitTorrent client (disable it in `about:config` by yourself), Opera Link that only merge two bookmarks together, requiring me to manually delete all default bookmarks before (or after) syncing, etc.
But it is a really good browser: fast, has minimum resource usage. I wish there is Opera Lite. (And hope Opera team to finally fix Asian language support.)
Still, I don't understand what benefits are to explicitly disabling Opera's built-in torrent client; why not use something else?