There's another nuance that should be noted.
As a series ages, there are changes to culture and a generation of people comes and goes. The series stands as a landmark to a certain point in time, a social way point if you will. When looking at TOS, it captures so many elements of the late 1960's, as well as their version of the future. Clearly the later Star Trek series have elements from their respective times, including a revised future that deviates a bit from TOS.
When I watch TOS, I can't measure it against the productions of today. They DO NOT APPLY. Not only was it a different era, with vastly inferior SFX tools, but also a lesser budget and more restrictive set of broadcasting company policies. What we ultimately have before us is a time capsule. A work of art. Something completely unique on its own. Whether or not you enjoy the stories, acting, or production values, you should appreciate it for what it is, not to mention the fact that without it all of the other Star Trek series (TNG, DS9, VOY, ENT) wouldn't have existed. As such, I find that anyone who bashes TOS to be sorely ignorant and if unable to convince from a single posting, not even worth my time debating with.