Man, I really miss the skins in Opera 12, there were some gorgeous ones. I agree that the email client is not necessarily a priority, however
> Every other major browser opens the link in a background tab, keeping the focus in the tab you're currently on.
Oh wow, that's pretty bad for every other major browser. We have two use cases:
1) I want to open the link in a new tab and switch focus (presumably to read right now).
2) I want to open the link in a new tab and not switch focus (presumably to read later).
In Vivaldi you can achieve (1) by "open link in new tab", and (2) by "open link in background tab".
In Chrome you can achieve (2) but no way of achieving (1) without touching the keyboard. And even when using the keyboard the shortcuts are clunkier (ctrl+shit+click vs shift+click in Vivaldi), which is probably not even relevant since I regularly observe Chrome users (programmers nonetheless) using "open link in new tab" and then clicking the new tab.
We should keep in mind that most browser users are not power users. Which leads me to
> but Chrome has acclimated us to some slightly different default behaviors
It's the wrong default behaviors, and ~8 years ago I could have listed you a dozen epic failures of Chrome/Firefox usability. I sincerely doubt there's some elaborate reasoning for why they behaved badly, most likely it was just inertia and "works good enough". One of my pet peeves is that Chrome/Firefox don't restore your last tab session by default, which I find extremely annoying: a regular user who doesn't even know how to open the browser options will always have to reopen the previous tabs when restarting the PC.
Nowadays I rarely use Chrome/Firefox so I can't make a detailed browser comparison anymore, but from what I've been seeing there's no reason to think they're anywhere near as good as Opera 12 used to be. I agree that the usability changes could be a shock for new Vivaldi users but it shouldn't be too hard to adapt to a better way of doing things.