My family lost our minds when Pelia (2X03) described the Federation as a...
spoiler
moneyless socialist utopia
I mean, we all know that's what it is, but I'm pretty sure that's the first time they've said it straight out in universe. And then the plot reminds of it several times as that very thing is put in danger.
there were a couple of times on DS9 when Quark and Nog flippantly refer to the Federation in similar terms, but they’re quick, throwaway lines and easy to miss.