Should I start watching what's already come before? I saw one or two episodes and thought it was pretty good, but nothing that great.

I watched Community on Turings recommendation, and that's utter pish.

You absolutely should. I got hooked pretty quickly and went through the entire DVD boxset in a little under two weeks.

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Gradually decreasing the quality of my own obituary.

Massively self - referential, and some of the jokes only make sense when you've seen later episodes, or build and build across a series, so it's one of those shows that gets better the more you get into it.

Is it quite dark? The one thing I hated about Comminity was that it was never dark, was all so bright and shiny and hahaha jokes every 15 seconds. If you are making jokes every 15 seconds, they aren't going to be funny.

I wish American comedy would learn this.

no it's not like that. it's very intentionally weird, works on paradoxes and absurdities, there aren't proper ''jokes''', everything is like an in-joke, mostly self referential (as they've told you before). It doesnt get 'dark' but im not sure what you mean by that.
the writing was amazing and the cast is just perfect.