As Jarno Peschier’s answer says, the brief for Marc Okrand was to create an aggressive-sounding language, that would map onto the “Blakh Vakh Gakh” aggressive sounds James Doonan had made up for the first Star Trek movie. And Okrand accordingly went shopping for gutturals: /x, q, qχ, ʔ/ <H, q, Q, ’>. I guess you can add /tɬ/ <tlh> as an honorary guttural, because of its affrication.
Does a language full of gutturals have to sound aggressive at all times? I’m sure Tolkien would say yes—which is part of the reason I haven’t gotten into Elvish. (Cellar door. Pfft. That’s just effete.)
Well, look at languages that have one or more of those gutturals. Is it possible to speak Arabic without sounding aggressive? Chechen? Nahuatl? German?
The human spirit, much like intonation and pitch, is suprasegmental. A couple of gutturals aren’t going to make a mother’s lullaby sound any less soothing to a baby.
I wrote a Crown of sonnets 22 years ago. It was a love poem sequence. The frontispiece was in Klingon. Here’s my reading of it. You tell me.