While, particularly for hard science fiction, Star Trek might not be the best example to follow it does have some things that almost every Sci-Fi writer can learn from.
One example of this is Vulcan’s, they are one of the more interesting fictional races of any science fiction setting. For me at least this is in large part because they feel complete and real in ways that say, Klingons don’t.
While you could give a short description of what they are, “They follow logic above all else and are emotionless.” That is only the surface.
You learn they DO have emotions, they have emotions so strong they nearly destroyed their civilization. They work all their lives to suppress them. Watching Spock and Sarek you see they do allow SOME emotions to guide them. They live in a paradox, trying to solve a problem they can never solve.
They believe in logic above all else, yet their culture has an element of mysticism. They believe in the immoral soul, a katra, that can be transferred from one person to another. Spock more then once bent logic to justify taking a moral action, such as risking himself to save someone who was almost certainly dead.
They feel complete because they are full of paradoxes and history, like any real people.