And most of all: being able to effect that responsiveness also manually.
The resolution(s) are getting higher and higher all the time, so it's not just a matter between the computer, tablet and mobile. It's also about many different type of screens and resolutions as well – and having the site looking IDEAL in all of them.
In my opinion this would mean the possibility to measure the responsiveness in pixels, percents and to have fluid, breakpoints both horizontally and vertically, plus automatically or manually.
And different combinations of them.
There are more than 100 million Wix sites, so there must be quite many exotic screens and resolutions where to look at the sites as well ;)
It would be a HUGE thing to be sure that your site is seen similarly in all of them.