Others have mentioned the nonlinear ordering of comment threads within a post, but I think the linear ordering of posts on one's flist is also important. It makes it easier to catch up, gradually, when for whatever reason one falls several days behind (as I have, now): one can just read with ?skip=... for a while, gradually reducing the skip until caught up, rather than having to remember what threads one was interested in and find one's place in each of them. Additionally, it's much easier to create a new post on livejournal than it is or was to create a new thread on Sugarquill.