Express knows where its "opinion" ends and where it should make room for other modules with opinions of their own.
There really are two opinions at play here. Opinions of how to do something and opinions of what to do.
Express has opinions of how that differ from the http module (it even uses the http module to accomplish what it does).
Express has a minor opinion on what it should be doing and extends the what beyond the http module, but that's largely where it ends.
The problem with Rails is that it the domain of subjects it had an opinion on kept growing until it couldn't not have an opinion on stuff.
CSS versus SASS? I love Sass and used it before Rails added it. Rails didn't need to have an opinion here. Coffeescript? Same thing.
Maybe it is an unfair comparison because there's no equivalent to this much opinion about so many things in the Javascript world, except some Rails ports that people have made. And even they are seen by most as "Oh that's nice that you can get set up quickly in less than a day, but no thanks, I'd like to pick and choose what makes sense to me based on what I'm planning on building."
Had we known better maybe we would have gone with Sinatra back when we started.