Personally, I think Star Trek went off-track when it began to make allies of enemies like the Klingons, and created new ones to replace them.
The Klingons, especially, would have made good continuing bad guys for the Star Trek series. They are almost polar opposites of the Federation: violence is the answer to most problems for them, their laws are most often draconian, and the lives of non-Klingons aren't important to them. But then, the producers and writers began to make the Klingons ineffectual and bumbling. Didn't it seem like Worf, for example, was always getting his butt kicked? Didn't it seem like Klingons were dumbed down in later series and movies - the original series showed smart, ruthless Klingon leaders who backed up their threats. Later Klingons were often a lot of bluster, with nothing to back it up. Plus, they managed to blow up one of their own moons, dooming themselves!
The Romulans were another good foil for the Federation - they were literally the evil twins of the Vulcans. The sheer narrative potential of the Romulans is enormous: they give a glimpse into what Vulcans once were, and may still be, beneath the veneer. But then the writers and producers turned them into something akin to the Snidely Whiplashes of the galaxy - they show up, do some bad stuff, and twirl their figurative mustaches. The whole Vulcan/Romulan conflict is largely ignored.
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