When a few teammates and I interview folks, other than the usual official evaluations, the best we understand each other's feedback is through this common language. A language that got unconsciously developed while interviewing and working together for years and our experiences with other folks in the organization.
We are no YC, but what PG is referring to as earnest would be in our language simply "good energy". The opposite of it would be "looks tired" i.e. for someone showing completer lack of interest in what they do. Another ones would be "fighter" (one who doesn't gives up on hard problems) and "pacifier" (one who stays calm and composed that's good for handling production incidents and help team through with difficult people).
To further muddy the waters, I think if the job descriptions came up with more details about the personality traits required for the job, that could help people have better job matches and satisfaction. I know it becomes too subjective if you overdo it but perhaps things like StrengthsFinder can help. On the other hand there's a risk of going too further on altogether a wrong path as it happened with Myers-Briggs.