Melodrama is low, goofy sci-fi hijinks are high, camp is off the charts. There's a lot of morality tales and vague (and not so vague) inferences about politics and social stigmas. Up until DS9 (and even for a large portion of that) the series are very episodic, rather than serial, so long-term stakes are generally pretty low. They're good shows to throw on occasionally if you want something relatively low stakes to watch but enjoy being forced to think about things.