I'm curious how it is decided how many comments to display before the "Show more comments" link.
Sometimes I see a dozen comments on a question and no "Show more comments" link, in other occasions no comments are visible at all, only the link.
On main sites, on most questions, it seems to be the best five (by votes) or the first five (if there aren't any votes). On meta sites, this threshold of 5 comments is increased to 15 comments.
On all sites, if the question has more than a certain number of answers, all comments that don't have any upvotes will be collapsed, and then the above applies to upvoted comments if there are more than five on a main site or 15 on a meta site. This threshold is set to 30 answers on most sites but can be changed on specific sites. For example, on The Workplace, the threshold is only two answers.
According to a January 2011 comment by Shog9:
it's configurable, per-site (meta is 15), and also changes slightly if the number of answers exceeds threshold (30, except on SU/P.SE where it's 15) in that all comments without votes are hidden.
As an aside, upvoting comments does not change the sort order of comments, but when there are lots of answers, or lots of comments, then comments without any upvote (or comments with less upvotes) might be hidden, until one clicks that "show x more comments".