I wonder if people treat comments a bit like search results? They will read the first one or two pages and stop after that (assuming the comments are paginated). If comments are posted in chronological order (oldest first), then you get the "click bias" that other people mention. Interesting comments that fall further down the discussion thread get buried because few people browse that far.

Also, if you have threaded discussions, a good post in response to another comment may need the other post(s) to provide context, so will simply highlighting that indivdual post make sense without the others?