A semi-objective thing about most of the uninteresting, non-HN-appropriate stories is how predictable they are, i.e. how repetitive of previous instances of the type. The more predictable a story is, the less information it adds and the less interesting it is.
There's a reason we say "intellectually interesting" to describe the stories HN wants. There are different kinds of interesting. For example, we all have an interest in the things we're personally invested in or identify with, but that is not intellectual interest.
Similarly, as humans we're highly interested in what other humans are up to, but that kind of social curiosity—think gossip and celebrity news—is not intellectual interest either. So a story can be interesting without being a good fit for HN, quite apart from not all having intellectual interests in common.
Maybe this explains why upvotes alone don't work for a site like HN. We upvote the things that excite our interest. It's too much work to pause and ask ourselves what kind of interest we're experiencing. And the non-intellectual kinds have an advantage—they require less attention to get excited. (For one thing, they rely on recognition, while intellectual interest relies on curiosity, which is an absence of recognition.) This is compounded on the internet, where attention is fleeting and fragmentary.