Thanks. May I ask what's the differences?
It's really easier if you just download and try it. You mightn't miss anything, or you might miss an obscure little checkbox option I would not be able to tell because making a table with every feature/option in Opera 12.x would be totally time consuming and not worth it. There are new features like Stash, some signature features (Speed Dial, Off-road/Turbo, etc) and other small ones were already developed into the new browser, while others are under development (e.g. sync), some work differently, lack a characteristic or changed places (the Recently Closed trash can menu is in the main menu now, some settings in other places, etc) and others we know that won't come back (or don't know whether will come back someday or not).
You may be interested in a comparison I made in this topic on Opera 20 vs. Chrome so you can see benefits that the new version still has over it and other browsers (of course it's not a complete list and may not represent fully the latest version available at the time you read it).
I may have discovered what was causing this. I use Ghostery to block a lot of ads and things of that nature, a few of which I unblocked for testing a website. I blocked these again and the box seems to be gone for good... hopefully.
There's really nothing you can do here. The views under "All Messages" are hard-sorted the way they are. Even if you try to change that in index.ini in the mail folder, Opera will revert your changes on you.
IMAP folder views and label views etc. are sorted alphabetically (simple ascii comparison) in ascending order.
For labels though, you can prefix their names with an "_" for example to get them to show up above the "a" labels for example.