This show has been a huge part of my life since birth. Being born in the late 70s, I grew up on reruns, comic books, and even had records of stories (with really bad "sound alike" voice actors). It was my first real introduction to sci-fi, even before DOCTOR WHO and STAR WARS. The writers used this show the way every great fantasy writer does, which is to use a fantastical setting to discuss real world issues, which causes the viewer to actually see what the subject matter is and learn from the lesson (or not, as we see every day in our modern world). Roddenberry wanted to create a WAGON TRAIN to the stars, and it certainly works. Equal parts sci-fi/fantasy, western, philosophical treatise, and political study. It captured the time it was created in, and yet presents a future that all humankind should be striving to achieve.
That being said, it's not perfect. Far from it. When people bash later incarnations of TREK and talk about how it doesn't meet the brilliance of the original series, I feel as though they should take off the rose-colored glasses and re-watch some of those episodes. I'm taxing my BRAIN as to which episodes they might give a SPOCK'S look at, but I'm certain it will come to me.