"Balance of Terror" established the notion that the traditions of honor that the Commander and Centurion valued were considered out of date in the Empire and had given way to a younger, more self-serving generation. So there's canonical support for the idea of Romulan culture changing and moving away from its traditions of honor. So that transition never really bothered me.
Really, it shouldn't be surprising at all if a culture's values change from one century to the next. An American from 100 years ago wouldn't have recognized modern American values: a black president, women in positions of authority, gay marriage starting to be legalized, America as a global power rather than tending to its own affairs, etc. What would be implausible is if a culture were exactly the same in the 24th century as it had been in the 23rd or 22nd.
And it's not as though the Federation didn't change during that time. The Starfleet of Kirk's five-year mission was drastically different than the organization served by Picard and Sisko during the twenty fourth century. Kirk's time reminds me of the Old West. Starship captains were left to their own devices when solving a crisis, which meant less debate and posturing and more action.
Things started to change around the time of the Project Genesis fiasco. Starfleet became much more self-conscious about its public image. Whether this was due to the Klingons saber-rattling or pressure from Federation bureaucrats, I don't know. But Starfleet was a much different organization by the end of Kirk's time aboard Enterprise
than it was at the start. Some of the changes were brought about by members of Kirk's own crew and set the stage for The Next Generation
, an era dominated by bureaucrats and self-important admirals more concerned with the letter of the law than they were with the welfare of the general public.