I have the same reaction to the new design. Another aspect that makes it even more maddening is that Canvas now puts additional posts on a second page, so you have to click through to see all the posts. Each time you click from one page of posts to the next or previous page, you have to expand all the threads again. It is extremely aggravating and adds significantly to my work load (not like I don't have other things to do).
As someone pointed out in a different thread in this forum, SpeedGrader doesn't show who the student responded to. So here's a scenario I've encountered repeatedly just grading one discussion:
I need to check who a student responded to. I go from SpeedGrader to the discussion (open in a separate tab). Must Expand Threads before I can see replies. Search for the student. No results. Now I have to go to the bottom of the page, click on "2" for the next page of posts, Expand Threads again, search again. All this for something that took a single search in the old Discussions. I'm having to do this multiple times for every discussion I grade.
I don't understand what logic there is in not having all posts on a single page, in addition to the points already made about expanded threads.