The issue with the former is that it relies on upvotes of people from the community. This leads to the problem of people upvoting comments on topics in which they lack expertise (ie developers upvoting comments about astronomy that "sound right")
This results in a trend (on HN and reddit) where the most upvoted comments are comments that "sound correct", but would not hold up against scrutiny of people who have expertise in that area. I like that HN censors the upvote count, so we are forced to judge the comment on its own merit.
E.g. if someone versed in cryptography upvotes or downvotes a cryptography story/comment, that counts for more than someone random
And I only care because the expertise gap is really the only flaw in HN/Reddit style ranking. In all other ways, it seems superior.