Language, absolutely, and what violence was there was pretty shocking from what I remember, though it wasn't a constant focus (a lot of the show was much more introspective, but also dripping in what some would consider a very technological horror) - but even cable can do grisly violence these days. No explicit sex scenes/nudity (though there was a fair amount of sex, we didn't see every inch of the participants' bodies), but then, the Alien movies never had any real nudity until Covenant.
Thematically, though, it was very "adult" science-fiction and the tone certainly reflected that.