I've never used Slashdot, but I have just looked its homepage and its content is more like The Verge, Wired, or similar websites, i. e., generic tech articles,

I've never used Slashdot, but I have just looked its homepage and its content is more like The Verge, Wired, or similar websites, i. e., generic tech articles, but more oriented to techie people. HN is a lot more niche in different kind of matters, like finding obscure problems with a compiler, how a company is using some technology and why, or simply articles about work/life balance and what to do and what not to do to achieve a fulfilling life.