Trek has mostly used exploring space as a means of exploring what it means to be human. It's a pretty effective storytelling device, and it's one of the reasons, I think, that Trek can drift pretty far in the direction of fantasy without really damaging the basic premise. Everything ends up coming back to humanity in the end: Spock is the most human soul Kirk has ever known, Data is on a quest to be more fully human, the characters are always stumbling back in time to our own era or some historical period of interest to us, alien races are caricatures of humanity, Sisko's a baseball fan, Janeway drinks coffee, and on and on.
It's pretty light as far as the science is concerned, though maybe moreso now than when the original series aired. It's also pretty depressing in a sense, at least to the extent that we actually approach the Trek universe as a vision of the future (which I confess I really don't). Humanity puts its internal struggles behind it to explore the galaxy, and the galaxy just ends up being full of the same kind of centuries-old hereditary conflicts, cynical political manipulation and senseless bloodshed that dominate earth's history? What a downer! It's just like earth, really, but you need warp drive to get from one place to another.
To the extent that Trek is about the future at all, it strikes me that it's built on the basic idea that in the future, despite whatever technological advances may take place, humanity as we know it will still have meaning and still be significant. That's why everything turns out to be human basically, in a sense, whether it's the rock-chewing lava monster or the sentient holograms or the subspace-dwelling creatures or whatever. That point of view makes notions like transhumanism problematic: the idea that humanity will have fundamentally changed in the future is threatening to the Trek universe. In Trek, being human has inherent value. Even genetic enhancement is treated as suspicious and dangerous, because the idea of "building a better human" seems to devalue things like effort, individuality, self-reliance, etc. Perhaps it does, perhaps it doesn't, I'm not really sure. But in Trek it turns out to be a bad idea. You can't really improve on a person in Trek. People improve themselves in Trek.
There are many beings more powerful or more technologically advanced than humans, in fact there seems to be an entity or civilization of this type hiding in every other star system, but in the end humanity is the measuring stick, the reference point. Star Trek is oddly geocentric, really. In a sense, it provides a vision of the universe that really does revolve around us after all. Not literally around our planet, but around our notion of self. Even omnipotent beings like Q find humanity endlessly fascinating. The Borg are threatening at first because they seem to represent a negation of humanity's pre-eminence, but in fact we can turn them back into humans: we assimilate them.
Now, I'm a big fan of Trek. I just like it. Intellectually speaking, I'm not quite sure what to make of it, though. Is it naive? Vaguely sinister in a way? DS9 sometimes toyed with that idea. Is it just wrong in the sense that humanity as we know it may no longer exist long before we develop the technology to travel across interstellar distances? Or maybe not. Maybe Trek is right and there is something important about our experience of individuality, our silly games of darts and poker, our search for meaning. Maybe it will last longer than we think.