Comparing the time line of both the browsers (Opera 15+ and Vivaldi) and the features incorporated it looks like Vivaldi would turn out to be a good browser.
Some of the useful features still missing after almost 2 years with new Opera but implemented in Vivaldi in Tech Preview itself.
Site specific preferences
Import from Old Opera (including bookmarks)
Closed Tab trash icon
Bookmarks (Opera did few experiments with this one and the current one also lacks some options)
Will have to see how Vivaldi shapes in coming days and months...
Yandex browser has also done some nice innovations which is also based on same engine
Another competitor coming up is Spartan (or the new IE).
Back to Opera.... Another question to be thought about is... Has Opera set the priorities right? It's clear that new Opera does not want to mimic the old one but is something like closed tab icon that difficult to implement that it has not been considered yet?
Opera had a pretty good user base established over the years which could have carried onto new version with providing some most used features and not just stick to bare minimums which has pissed off lot of users to switch to other browsers.