It's always been my understanding that in TOS, the Klingons were supposed to represent the Soviet Union/Eastern Bloc, and the Romulans represented Red China (with the Federation of course being NATO). Despite all the college kids making excuses for them, the Soviets were a bunch of hyper-nationalistic imperialists who were constantly bullying and pushing around smaller countries, pretty much like the Klingons in TOS. I also think Pirates are a really good analogy for TOS Klingons. They did whatever they were strong enough to do and didn't care about any kind of rules or code. Meanwhile, the Romulans were supposed to reprsent all the paranoia surrounding oriental communism, with their isolationism and the fact that not much was known about their society. Mao, while not really any "better" than the Soviets, was a thinker and intellectual, not the kind of leader who'd shout "We will bury you" at a diplomatic reception. The Romulans were always more cerebral, and in TOS had the strict honor code demonstrated by the "Balance of Terror" Romulan commander letting his ship be destroyed rather than being taken hostage.
In TNG, they conflated some of these traits. The Klingons agression was offset by an honor code, and the Romulans, while still being cunning and intelligent, became complete scoundrels who'd do anything to win. I know some people on here find the Klingons pining about honor to be annoying, but I think these changes made them both better and more complete villains.