(1) get rid of those annoying UI clutter for options
(2) make the default keyboard shortcuts to be like FireFox and Chrome (yes FF and Chrome's Keyboard Shortcuts have become a "standard" in my opinion)
(3) Some really useful AddOns like AdBlock and FireBug for Opera
Then, I would be a converter. I feel like this Opera 10.5 is the very first version of Opera that I actually enjoy using for the first time ever!
Previous versions sucked and was not the right browser for me.
Options... great Scott its absolutely AMAZING how much functionality Opera truly has, I use it only as a browser, BUT if you actually look into it, it's also an E-mail client, and RSS feed aggregator, and a full features IRC client, AND a web-server called
Unite., all in under 10 MB.
So Opera sort of NEEDS that many options, to control all of its tightly integrated features. Who doesn't like visual tabs, AND a cool looking zoom slider on the bottom right of the Opera UI
For the AddOns like AdBlock and FireBug you request, the AdBlock feature you are used to is not auto-updated by a 3rd party list, instead you right click on a page and select "Block Content" which will bring you into an ad-blocking click session, to block
any element on the page to create your own custom blocking rules(which I love and use extensively).
The FireBug which you want is called "Dragonfly"(needs the normal Menu Bar enabled or some keyboard shortcut I can't remember to launch), its a thing very similar to firebug, which even allows remote debugging of a web page.
Also, the one advantage Opera has over Chrome is the "Flip" feature for tabs on your start bar, Chrome & Opera both have Jump-lists implemented but Chrome doesn't show your tabs like Opera can, but Opera, IE8 and Firefox x64 allow it.