How hard is it? Very hard! Every metric that might be useful to detect high quality content is then exploited and abused by SEO spammers to make it look like their own content is doing well by those metrics. They do it with fake accounts and outright abuse. They also do it with manipulation of human psychology through clickbait titles, so even restricting analysis to "trusted" accounts doesn't work because even the best of us are lured into reading crappy articles by clickbait headlines...