I lost all my open tabs in all workspaces with this update. I consider this a bug.
I disabled workspaces and now a solitary Aria button occupies the top half of my sidebar. Annoying.
And something that has been mentioned already: with the new interface, I need to carefully position my mouse in order to hit the scrollbar in a webpage. Just bringing the cursor to the right edge of the screen is not enough. This annoys me constantly.
Most importantly, Opera has broken bookmark extensions with this new interface. When bookmarking a page through V7 Bookmarks, for example, instead of bookmarking the current page, the extension gets the data from the active tab in the first workspace, and bookmarks that instead, unless, of course, I was already in the first workspace. (This is why I disabled workspaces btw.)