This implies that I have a good idea of what is good and bad language. I have. Language is an instrument for communication. The language which can with the greatest ease make the finest and most numerous distinctions of meaning is the best. It is better to have like and love than to have aimer for both.
In the book, he makes some effort to use this to judge various changes in English lexical semantics: he deplores the loss of the old senses of "liberal" and "conservative" caused by their adoption as party political labels replacing Whig and Tory, but regards the change of "wit" from "genius" to its modern meaning as having happily made it a useful word.
What would his reaction have been to some of the changes in English that have occurred since? Applying his criterion strictly, he should have welcomed words like "vape" or "twerk" - new forms expressing previously unlexicalized meanings. (His probable reaction to their referents is another story!) "Irregardless" should have left him unmoved - a new (actually not that new) word for a meaning already expressed by "regardless" has no impact on the ease of making "the finest and most numerous distinctions of meaning" (and may make it easier for poets to fit their thoughts to the metre). The use of "literally" as a general intensifier, on the other hand, should have driven him up the wall - he specifically complains about "verbicide" through inflation, citing the comparable case of "awfully". In brief, whatever the merits of this criterion, it cannot consistently be used as a general-purpose attack on novelties; it forces the prescriptivist to consider them on a case-by-case basis.
Assuming such a criterion is accepted, the next move is predictable: someone somewhere is going to want to compare the merits of different languages on its basis. The problems with that should be obvious. Suppose language A makes finer and more numerous distinctions of meaning in one semantic field than language B, but in another semantic field the reverse is true (as is usually the case). How do you weigh the importance of different semantic fields in an impartial way? To make matters worse, many of the relevant distinctions of meaning are only going to be familiar to a handful of domain-specific experts; can we really consider them as properties of the language as a whole (whatever that even means)? A criterion like this makes more sense as a standard for measuring individual changes than as a metric for comparing entire languages.