"Envious" has all but completely been absorbed by "jealous" in the vernacular. People just use "jealous" for both things. Good thing there can never be any ambiguity about which sense is intended.
"To comprise" is deployed incorrectly more often than correctly. Which is silly since, when used incorrectly, you could have simply used "to be composed of" (which is the thing people are confusing it for)—there's no benefit to using "comprised" there, all its elegance and subtle shade of meaning are lost anyway when you jam it into that clunky phrase as a perfect synonym for "composed". I think that's one of those fake-fancy abuses of language from business folks, leeching into everyday language. What synergy!
The apt adjective, rather than various words and modifiers expressing degrees of good or badness—many of which used to express more, but no longer do, as "massive" or "awesome"—seems to be an endangered species.
Awkward use of "less than" inexplicably replacing the word "inferior" in some circles. "We must ensure none of the children feel 'less than'". In the words of my generation: "LOL WTF?"
Anyone have handy any studies on vocabulary among Americans? It looks to have markedly decreased over the last few decades, but I worry I may be falling for the same kind of bias that seems to make everyone think everything's getting worse all the time. Popular writing gives me the impression it's written for an audience with a smaller vocabulary than in, say, the 1950s or 1960s, though.