That's why you don't let users invent tags of their own but enforce a unified premade dictionary.

Then it's still hard to make sure people use them properly and consistently; even with admin-specified tags, those tags can be applied inconsistently by post authors such as to deflate the specificity of the tags or shift their meaning.

Also I know at least one website which lets people with sufficiently high karma retag posts of others -- that should be helpful with fixing problems.

I think maybe the reason Luke is specifically suggesting changing the subreddit partition is because this requires a one-off, upfront, unusually small (compared to other LW improvements) investment of time. High-karma folk (re)tagging is riskier because it puts an ongoing burden on a small number of people, introduces more opportunity for drama and disruptive disagreement, and very probably introduces other issues.