Once there are more than threshold answers, all non-upvoted comments are hidden. This is done to save space, and happens across all sites.
On SU, the threshhold is 15, vs 30 on other sites. This was done in response to the huge number of non-questions on SU... So you're probably more apt to notice it there.
Note that this is also the threshold for forcing a question into CW mode, though questions can be marked CW without triggering the auto-hiding of comments.
###Evidence:
- A non-CW question with 15 answers: comments visible
- A CW question with 15 answers: comments visible
- A CW question with 16 answers: comments hidden
###See also: Reduce community wiki force threshold to <= 15 answers on superuser?
