^^^Funny, because the contrary to the opinion generally held in Hollywood: films are where you focus on spectacle and not character. Character you can do on TV week after week.
There's no essential problem with high-concept SF. As in any film, it's the mix that determines if it's good.
Spectacle really isn't the right word. Most Star Trek films have plenty of spectacle without detracting from the focus on the characters.
To clarify, I don't think the theme of exploring the unknown is generally well suited to films. Ultimately it comes down to the fact that whatever unknown that gets explored is no longer unknown by the end of the episode or film. Afterwards, it's just "there." The episodic nature of television allows us to move from one "unknown" to the next on a regular basis.
Putting it in perspective, it's generally held that Star Treks II, III, IV and VI are the best of the films based on TOS, while TMP and V are generally viewed as being poor by comparison. If there's something that II, III, IV and VI all share, it's that the characters aren't static. They always finish the movie in a different place, personally, than when it began. While it could be argued that this was also the case with TMP, it's only to restore the status quo that was tossed aside when they promoted Kirk to give the film a secondary conflict. And in V, the characters don't really evolve at all. Even Bones, with his big revelation of having euthanized his father, doesn't end up in a different place. Don't get me wrong, the scene builds his character fantastically, but it doesn't really affect it in the end.
What makes II, III, VI and VI so special is that they dared to move the characters, and in doing so, dared to move us. TMP said "here's a space cloud looking for God," and V said "here's a crazy Vulcan looking for God," and then basically simply had the characters along for the ride. These would have made great themes for Star Trek episodes, but they don't hold up as being movies.