For us older fans (I'm 62), I think one reason TOS has such a warm place in the heart is it's what started it all, and without it, none of the later series or films ever would have existed. There's a sense of comfortable familiarity with it, because we went through the 70s watching those 78 (or 79) episodes over and over again, five nights a week, during its syndication days. I tend to enjoy watching the TOS episodes more than the later shows, because of that familiarity.