A couple of things I really like - (1) that the browser chrome changes to suit the colour scheme of the website that you have loaded when flicking through tabs. Makes it really easy to identify what site you are on especially if you have scrolled down past the header. (2) that they have a slider on the bottom toolbar for zooming in/out of websites. For this old guy with failing eyesight, that is a killer feature that I use all the time.
Happy to also see that the number of sites reporting "You are not using an approved browser" has reduced to nearly 0 when browsing via Vivaldi, but that is more a problem for lazy web designers who only create sites specifically for a particular rendering engine, I guess.
EDIT: Interesting Observation - I remember that in the early days, their main selling point was their command line functionality for power users. That was what made me change over back then, although ironically I never ever use their CLI tool set at all these days, and just use the mouse like always.
I note that they have downplayed the CLI functionality on their current website, so perhaps it wasn't such a drawcard for other users either?!?