There's a continuing theme throughout about how to react with courage and resourcefulness in a seemingly hopeless situation.
Phaser and photon-torpedo battles, with deaths (including major characters in the series). Two men are tortured in gruesome detail with brain-burrowing insect-like parasites put into their ears. Scenes of corpses after a massacre.
We learn that Kirk has an illegitimate son by an old lover, and she describes the Starfleet hero as "no Boy Scout." Kirk eyes a pretty Vulcan, but that's as serious as it gets.
"Hell," and various forms of "dammit" from Dr. McCoy especially.
Star Trek is an enormous commodity with plenty of merchandise available to buy.
Social drinking, with Dr. McCoy bringing Kirk some "old" (22nd century) wine for his birthday.