This is all bourne out with what we see of them on-screen in The Original Series. Klingons are repeatedly willing to create conflict where ever they go, despite the alleged Organian Pace Treaty with the Federation. They are also duplicitous. They'll say one thing then do the opposite. Romulans, on the other hand, are often shown as honorable opponents. The Romulan captain seen in 'Balance of Terror' is ruled by his own sense of integrity, and shows great pride and a distinct code of honor. The same can be said of the female captain in 'The Enterprise Incident'. She offers Kirk and Spock the chance to explain themselves, but treats them with dignity and respect (even as they outwardly treat her with none).
By the TNG, these traits seemed to have been flipped. The Romulans were traitorous even among their own, highly secretive, and often played other species for their own benefits. As Lieutenant Worf repeatedly tells us, "Romulans are WITHOUT HONOR". The Klingons in TNG are effectively ruled by ancient codes and ethics that they were duty-bound to uphold (even if, again, behind closed doors they often don't, cf. the Duras saga).
What do you guys think? Do you reckon the TNG writers room misunderstood these races as they were portrayed in TOS? Was Moore pehaps wrong to have swapped their attributes? Or do you think it was maybe a good thing, to show these races in a different light than TOS portrayed them?
You're right that the Klingons and Romulans had pretty much undergone a reversal by the TNG era, but it doesn't make sense to blame Ron Moore for that, since he came along after it had already happened. It began with The Search for Spock
, in which the villains were originally written as Romulans -- flying Birds of Prey with cloaking devices and talking about honor -- but then got switched to Klingons without the script being substantially rewritten at all. That was the beginning of the association with Klingons and honor.
Later, when TNG came along, Roddenberry initially resisted reusing any TOS races, but his co-developers convinced him (IIRC) that he should include a friendly Klingon among the crew in order to show that the Organians' prediction that humans and Klingons would one day make peace had come true. So Worf was conceived, and episodes like "Heart of Glory" and "A Matter of Honor" would flesh out the idea of a Klingon culture that was admirable because of its commitment to honor. By the time Ron Moore joined the staff in season 3, that redefinition of Klingon culture was well-established.
As for the Romulans, "Heart of Glory" established in passing that Romulans had killed Worf's parents, and the rest seems to have grown out of that -- it led to the writers giving Worf a grudge against Romulans, and since he and his people had come to be defined as valuing honor above all, it followed that he would consider the people he hated to be without honor. As for the treachery the TNG Romulans did, in fact, show, that may be partly influenced by Worf's opinion of them, or it may have simply been because, with the Klingons friendly and the Ferengi a bust, the Romulans fell into the role of the series' primary villains and thus had to be evil enough to fit. But again, this was already established by the time Ron Moore came along. He didn't create it.