I just finished the first disc of fandom's favorite season of TNG, season three on Blu-ray and out of five episodes only found one remotely watchable (The Survivors).
Do I view TOS through nostalgia-tinted glasses? I'm sure I do. But even when it's bad I'm never bored. I can't say the same thing about Modern Trek.
I love the nostalgia, and TNG and DS9 were what started my decline in Trek viewing.
Much has changed in 50 years -- editing styles, acting styles, directing styles, writing styles, technology, hair styles, etc.
TOS has a lot of "warts," even by the standards of the 1960s and 1970s, but it remains my favorite Trek television series.
Quite agree, look at contemporary shows, from Wild Wild West to Gilligan's Island and you'll see that it was made to the standards of its day.
Bear in mind that's it's almost 50 years old, how many other TV shows from that era are still shown in re-runs? The fact that it varies in quality means very little, even modern TV shows can vary in quality from episode to episode. As for pacing, sometimes fast is good, but equally sometimes a slower pace can be good.
How many shows are in reruns from the 90's, much less the 60's? Some folks enjoy entertainment from other eras, while others don't for various reasons. To say something ages badly? That is often a personal dislike trying to mask itself as a fact. Everything ages even kids of the 90's.