"High quality comments" for a user in general is completely subjective. They might have "high quality comments" on C programming, and "low quality comments" on

"High quality comments" for a user in general is completely subjective. They might have "high quality comments" on C programming, and "low quality comments" on marriage equality. You'd need a custom filter for every possible topic of conversation, and even then you'd have to filter based on what you find to be "high quality". It's easier to just filter based on popular opinions or popular people, which is what karma is for.