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Take the natural logarithm of the comment karma, and then use as the weight in a weighted shuffle algorithm?

Higher-voted comments will then be more likely to move up, but not guaranteed.

(You might want to consider putting a thumb on the scale to distinguish between comments which are low voted because they're uninteresting, and comments which contain flame-bait words etc.)

Maybe some per-user jitter in the ordering so that different people are seeing very slightly different things at the upper end, giving those posts the chance to be seen?

I like the comment collapse idea but i think it would be worth having a sorter so you can see the top comment thread but also newest first and most up-and-coming threads.