Hacker News - The ``hacker'' part is a joke. It's more like startup/yuppie/web-designer news. Actual hacker news gets ignored and hacker humor gets downvoted. Most of the headlines are linkbait and read like articles in Cosmo. It's so bad that articles named ``10 Ways To Do X'' must be renamed to ``How To Do X''. Most of the submissions are just are blog posts addressing whatever topic is dominating the front page in hopes of getting lots of hits. The community is shit.
Slashdot - Much better community, but the hackers there are mostly aging cubicle code-monkeys. The news there is not necessarily technically, just things that autists find interesting: Star Wars, astronomy, dinosaurs, etc. A lot of interesting stuff never makes it past the Firehose, but who-gives-a-shit discovery on Mars makes the front page.
/r/programming - I've never lurked there, but apparently Hacker News was built in response to proggit, so assume it's just Hackers News with less humorless anal Steve Jobs wannabes. Whenever I've seen it, it has most of the same stories are Hacker News with the exact same headlines, so I usually just close the tab. /prog/ - Pure shit. Most of the people here can't code. The ones who can are dead end programmers: they don't have the creativity to make cool shit so they learn toy languages as a hobby. This leads to a lot of stupid language wars. Show off something you coded and the first thing they'll do is check to see if it's coded in Ocaml. If not, prepare your anus for autistic rage.