I do understand where they are going - for new users in particular, you want to profile prominantly highly rated comments to draw people in.
However don't forget your active and existing members! If I set my preference for oldest first, remember it!
And even for newbies, there needs to be an indication of context and hierarchy. As was pointed out, it's confusing all the time when first looking that things are out of order. There need to be visual cues.