Well, I am a hard sci-fi fan - and, if you would like to know my perspective - I have never once dismissed Star Trek's value to the science fiction genre - I see it as an endlessly interesting source of stories - and it directly inspired me into reading hard sci-fi.
I don't know whether people really believe Star Trek was simple, or whether people are underestimating it, and thus thinking "nah, they can't possibly have been that clever in the 60s" - it's hard to tell. Look at 'Where No Man Has Gone Before', with it's exploration of whether a human is still a human, a friend is still a friend, after gaining the powers of a god - if people underestimate the intelligence of 60s screenwriters, they could be forgiven for thinking Kirk's/Spock's lines about "that is not your friend / he is not Gary Mitchell" are throwaway lines, maybe just a justification for the death of Mitchell - but they actually could be deeper explorations of concepts of object/essence (existential philosophy), and the nature of power (does the exposure of a normal healthy personality to the power of a god, change the fundamental essence of that being, beyond recognition, even without any moral failing on the part of the individual - i.e. is exposure to ultimate power a problem in itself?) No philosophical concept is unworthy of exploration in my books.
Another example would be 'The Cage', and the exploration by Pike and the Talosians, of what part of the personality wants what is actually best for a human - which is most appealing; their id, their ego, or their superego (in Freudian terms) - or none of those - whether fulfilling them in a simulation is as satisfying to the psyche as the reality, and if not, why not? Pike is being tempted with a married existence, which the Talosians have plucked from his recent musings with the ship's surgeon as to the source of his growing sense of dissatisfaction, and then when this fails, they switch from superego to id, and hit him with the full force of a libidinous and megalomaniac fantasy, in the form of existence as an Orion slaver prince - that part of a healthy mind which desires control, domination, carnal pleasure, without any thought for the emotional needs of others, or the greater society. It's only simplistic if you dismiss these themes as worthless, or too prosaic - I don't think we should. It also has a deeply ambiguous ending, because for all Pike's rejection of the constructed reality of the Talosians, for how it fails to satisfy the psyche - Veena actually chooses it over her real existence, as a happy illusion in place of her horrifically deformed and painful reality - there is no absolutist conclusion favoring Pike's interpretation of the value of reality.
In my experience, people look at something with a certain philosophical outlook - it colors their perception - and this can literally render something meaningful meaningless, depending on whether the concept's contained in a work of fiction align - this is why people continually discover and re-discover different works of drama and literature over time - their outlook is re-aligning with the themes of a given story. You won't believe the amount of fantasy fans I have known who become enamored with dark fantasy and George R R Martin, only to rediscover the value and complexity of Tolkien - The Lord of the Rings was never simplistic to begin with; their approach to it just changed. I am not saying dark fantasy is without value either! Just different value! Star Trek, to me, is such a timeless work as Tolkien - but people need to be open to see it; or they will look for elements that disappoint, and inevitably find something.
Earlier in the thread people noted that many great dramas explored drug addiction before Star Trek - the suggestion being that an issue of immediate social and emotional importance, which is surrounded by prejudice, and strong opinions, is a worthier topic than traditional Star Trek fare (metaphysics, ethics, epistemology, the implications of technology, of science, cultural commentary, exploration of the mind, etc). I disagree. I don't think, as many do, that Battlestar Galactica, is necessarily more important than The Next Generation, just because it featured ethnic prejudice, deeply flawed characters, suicide bombing, and religious zealotry, and The Next Generation featured a functional society - in fact, some of the best works are those that dare to present a functional vision, in order to stimulate the imaginations of the readers - often they are the ones banned for being 'radical' or 'extreme'. People still endlessly analyze Shakespeare, despite thousands of dramas having covered the same ground - the reason being that just because something has been said elsewhere does not make it's fresh exploration any less interesting.