Year Outside, Hour Inside: Thanks to relativity, truth in television for lighthugger crews, who slowly build up a hefty time differential (and cultural delta) with the parts of the universe operating on the empire-time frame.
(Hint: if there’s a family member or social grouping you just can’t live without, make sure you all sign up together.
…also, give up on playing MMOs or multiplayer games with anyone off the ship, no matter how much tangle you brought. Timeslip is much worse than netlag.)
You Can’t Go Home Again: A consequence of lighthugger travel and the ensuing time dilation for ephemerals; or people from ephemeral societies, rather, since it’s the high cultural delta of the society you left that causes the problem.
Also, more rarely, what happens to people left off-world at the time of an Apocalypse How or Gone Horribly Right.
Multicultural Alien Planet: Well, I try. In practice, it varies by species, depending on how much time I’ve spent detailing them in particular, but they’re all intended to be this way, outside the odd Hive Mind, or other special cases like the Equality Concord.
As another note, this is mostly true of homeworlds, and true of colonies to a much lesser extent. While homeworlds have usually had plenty of time to evolve regional cultural differences in the absence of high-speed communications, later colonies have generally had those homogenizing their cultures from day one. Between colony worlds, though, you can find the same sort of cultural delta.