You can add "extensions" to the list of things missing from the new Opera. With the switch to Chrome/Chromium as the basis for Opera, existing extensions and add-ons have become instantly obsolete.
The new Opera doesn't support the old extension format and most of the APIs. It also doesn't support Chrome extensions out of the box.
That looks pretty bad at first glance, but there's a silver lining. While old extensions don't work anymore, since they're all written in standard web languages, they should be fairly easy to convert to the new Chrome-based format.
And, of course, Chrome extensions should be trivial to "convert" for the new Opera. For now though, the extension selection for the new Opera is abysmal.
"Opera 11 & 12’s extension format is no longer supported, and instead, we’ve switched to Chromium’s extension model. At this point, Opera 15 (available as Opera Next) supports a subset of the Chromium extension APIs — with more to come —, as well as our own Speed Dial API," Opera explained.
Old extensions can be converted
In fact, Opera has an automated tool that makes the entire processes of converting old extensions to the new format rather seamless, at least for some of them. Others, which rely on APIs no longer available in Opera, may require some modifications.
The tool is built straight into the developer dashboard of the Opera online extension catalog. All developers have to do is click Upgrade and test to see if their extension works with the new Opera.
Most Chrome extensions should work
However, because the new Opera is based on Chrome, extensions created for Chrome should work. Opera only supports a subset of Chrome's extension APIs, so not every extension will work, but most should.
Opera also plans to add more extension APIs of its own. For now, there's only a Speed Dial API, so the Chrome and Opera extension formats won't be completely interchangeable, i.e. not all extensions created for one browser will work in the other.
Overall though, the new Opera should run most of the many thousands of extensions in the Web Store, a good thing for users. That is, if the developers bother with uploading the extensions to Opera's extension gallery as well, since they won't work from the Chrome Web Store.
No Chrome apps for now
Note though that this only applies to extensions, Opera hasn't said anything about apps, neither the old but still in use format, nor the new packaged apps that can run in standalone windows even when offline.
The new Opera could, in theory, support apps, since the code is there in Chromium, but the company is likely focusing on more immediate things that need improving or implementing, for the time being.
No themes either
The new Opera does come with a few Speed Dial themes, but that's all you get, the built-in ones. There are plans to enable third-party theme support soon, but there isn't any at the moment.