That’s one of the most debated questions in terms of language, in my experience. It’s an important question because there is at least a general consensus that there is good language and bad language – acceptable and unacceptable language – and a common question because everyone seems to have an opinion on the subject, although they tend not to be exceptionally informed opinions. It’s also important because there are plenty of people – some who have knowledge of language and some who really don’t – who have decided at some point that they are the arbiters of what is good and true and what is not and dispense advice (often unsolicited) or make disparaging comments about language use, be it word usage, grammar, mechanics, or style.
I don’t consider myself an expert on language use by any means, but I think my interest in language is perhaps greater than the average layperson: I have studied writing theory, I have read grammar texts critically for personal edification, I regularly read blogs about language and try to keep up with what people are talking about regarding language, and I’m a certified English language teacher. I don’t claim that my advice on language is gospel, and I stress to my students that comments on written language especially are mostly tentative (even though I think it would be prudent for them to take my advice). Generally, I think I know what I’m talking about, but I’m open to correction from people who know more about the subject, primarily linguists.
This in mind, I’m pretty used to people making comments about language when they lack relevant training, like that old proscription against terminating prepositions. But I still confess that it puzzles me when I see people who are qualified in the area of the English language railing against things about which they really should know better.