House of Cards had the British version to build on, so it's not entirely a Netflix taste triangulation.
But, the CEO or somebody from Netflix has actually spelled out in the past that though they're taking risks in making a bunch of their own content, the risk is smaller than it has historically been. They know the taste of their audience (on a nearly individual level) better than any entertainment company in history ever has. And, they extract the value of their viewers directly and without middlemen, unlike almost every other entertainment company in history. So, for them, a successful show is one that brings in X number of new subscribers and keeps them around through multiple seasons...and that X number is smaller than it's ever needed to be.
So, you're right; they are triangulating in on very specific sub-genres and very specific audiences in ways that entertainment companies in the past couldn't afford to do. This is long-tail television production. Stranger Things, while it is a niche market, is actually one of the bigger niche markets they've tapped into, thus far. It's gotten far more buzz than a lot of the other Netflix original programming.
So, they're either getting better at the triangulation (likely) or they got lucky with this particular toss at the dart board of niche programming, or maybe it's a little of both. And, maybe the people making this show happened to be particularly passionate about it, and it shows in the attention to detail; they're having fun with it, and it makes it fun for viewers who like this sort of thing (or, are at least nostalgic about it...I don't read/watch Stephen King novels/movies very frequently as an adult, but loved them as a kid).
I find the quality of the Netflix originals to vary pretty wildly, and I don't get hooked on most of them. But, I enjoyed Stranger Things. It's a fun escapist bit of entertainment. It's no House of Cards, or even Orange is the New Black, in terms of quality, but it delivers very effectively on what it promises.
I wonder how well Stranger Things is doing among viewers who aren't old enough to have grown up surrounded by this stuff? I know many of my friends on facebook are hooked, but mostly the ones really talking about it are around my age. It's an interesting phenomenon, I think; the stuff it is paying homage to and modeled after was mostly targeted at kids and young adults, but the people most excited about it are maybe middle aged. Weird how that happens.