At this point, I just can't do without Stacks, Tile view, Sessions and Work Spaces. The build-in Periodic Reload is also great, but by far, the most incredible feature is that you can customize your right-click menues however you like. I also have a lot of Command Chains set up in the Quick Command bar.
The amount of stuff that you can customize in Vivaldi has been without parallel for me.
I had no idea that it was a Dutch app! It's really great, I've been using it on and off for a year now to great success.