You're right, of course, but the case has already been made in just about every freshman psychology textbook.

It is reasonable to assume that an educated member of the general public will be familiar with the Big Five personality traits, and that a typical LW reader will have noted and remembered the Big Five as one of a handful of psychological 'findings' that actually do have demonstrable explanatory power. There is moderately compelling evidence that they are, e.g., heritable, that they can be affected by drugs, that they correlate strongly across cultures, years, and survey versions, and that they can be used to prospectively predict various mental disorders and success rates. The Wikipedia article http://en.wikipedia.org/wiki/Big_Five_personality_traits does a reasonably good job of documenting this. The Big Five are also the lead feature in Chapter 2 of "Psychology Applied to Modern Life," (10th ed.), which I believe has been strongly recommended on LW before.

No doubt you meant well, but by suggesting that gwern used a "fuzzy label that one likes to attribute and then rationalize as having explanatory power," you also suggest that gwern is indulging in sloppy mental habits and that gwern's article lacks an appropriate foundation. Neither suggestion is really fair; the Big Five personality theory is so well-established that there is nothing improper, even among us skeptics, about relying on it and building on it to make a series of interesting points.