As a quick example, “My cat is making a coughing sound, help” is for General Discussion, while “Let’s talk about cats” is for Generals.
Given that threads are expected to be larger than average and carry on via the inertia of the grand topic they deal with, “sillyness” is fine as long as the whole thing doesn’t get permanently derailed or turned into a nonsensical mess. The general site rules still apply; don’t call someone “a cuck” because they like Captain America more than Iron Man, post illegal content, etc.
Regarding NSFW, use spoiler tags or site >>#t/p syntax, and give content warning (ie. blood and guts, or butts, whatever). If the entire thread is meant to be NSFW, include that clearly in the topic with a [NSFW] preface and something descriptive enough; ie. don’t call it ‘[NSFW] GOOD STUFF’ and then it’s a thread of just foot fetish art. And still follow Rule #7; if your gonna post boobs in a grimderp thread, spoiler it.