Old.reddit UI fills the entire width of the screen, and default view is compact. You see more of the feed at a glance without having to scroll as often. When it comes to comments more of it is displayed when it comes to nested comments before clicking.

New reddit takes up a lot of space. And clutters up the side with stuff like related posts when you check the comment section.

Another big one is reddit enhancement suite which lets you tag and block things only works on old.reddit.