I use classic mode and perhaps have some setting that preserves some older layout. The thing that I like about the older layout is that I can get a real sense of the conversation that is taking place by seeing the hierarchy of comments. I may only be primarily reading +4 and +5 comments, but I can instantly see where they fit in the conversation. Perhaps someone posted a +5, was replied to for two levels, and then someone posted a +4. Showing the hierarchy allows me to instantly understand where the +4 was in relation to the +5, and if I wish, I can quickly open one of the intermediate parents in a new tab, allowing me to view its siblings as well.
This works well for how I read slashdot - I scan down the page for interesting comments, and then open some of the summarized lower level children in a new tab and read through them. If someone posted an interesting point, it is easy for me to view all of the replies.
Admittedly, I have not used the beta much, but I have not been able to figure out how to read in my style. Say that I am reading at the +4 comment level. I have no way of discerning if a particular child comment is replying to the +4/5 parent above it, or if there are several intermediate posts. I have no way to see all of the replies to a particular post.
My experience with classic mode is that I'm reading a conversation, and I have a choice of where I want to dig in further to the conversation. My experience with beta mode is that I'm just reading a bunch of random comments with no good understanding of their relation to each other.