Comment collapsing is a useful feature. The problem with giving the largest weight to the most upvoted comment is it also gives a large weight to all replies (unless they're heavily downvoted). It's useful to be able to skip those replies once they get less interesting.

However, this does lead to yo-yoing: reading comments downwards, going back up to find the right collapse button then reading down again. It'd be useful to have a more direct "fast forward" button. Though admittedly I haven't thought this through in terms of UI design.