There was a Captain's Yacht? Too bad, then, that we never saw one! Not until ST:INS, that is.
TNG defined the Klingons for us, inventing the Klingon lifestyle, form of government, biology and history where TOS had none yet. TNG also defined the Romulans; although there had been a tad more groundwork done in TOS, it had not created much of a definition, it had merely created (welcome) diversity through the two very dissimilar episodes that actually featured Romulan characters. Both species still remained generic bad guys without distinguishing features (other than the pointy ears on the Romulans) until TNG made Klingons the drunken boasters and Romulans the scheming totalitarians.
...Hey, I didn't say those would have been complex or intriguing or attractive definitions!
The Klingon definition had time to make it back to the TOS universe, too, although "Council Leader" became "Chancellor" and "Homeworld" became "Qo'noS"; bigger budgets added to the level of detail in other respects, but it was nice to have such additions in the writing, too.
Timo Saloniemi