The most major thing I am missing is the developer tools. The one that is currently included is a slightly modified Chrome web inspector that isn't as functional as Opera's Dragonfly was. I'll probably be able to learn to use it, but I am going to miss the features and ease of use that Dragonfly offered, especially minor things like being able to close it with CTRL+W, without closing the tab.
Another big feature is the tab management settings. Currently there is no tab stacking (grouping) and no pinning, both features that I made use of quite a bit with Opera. Pinning will probably make it into a future version, but I'm not sure whether tab stacks will, and that is one of the handiest features if you keep a few hundred browser tabs open at a given time.
There is an obvious difference in speed however. The Chromium based Opera is much faster, and rendering is quite a bit better. I can also use sites that use to block Opera easily enough such as Google Docs, Google Music and Asana. Still doesn't make up for ruining my favorite browser .