The new layout...
I think perhaps it makes more sense for new users. But it's not like the old one was particularly complicated - it just took a little getting used to.
I'll echo what others have said and say the old one was better. I think, especially for a rather small community like this one, the old paradigm worked better and that's why I didn't mind the non-traditional layout. It was easy to check in and immediately see what had changed since you last visited.
I also liked the "non-flat" (it was pretty flat anyway iirc) layout better. The "flatness" seemed to be purposeless and in many places it lacked contrast where the other theme had it.
Also - it could be something else running on my computer at the same time or heavy load or something completely unrelated - but as I'm typing this post now, the editor is super laggy. I can type faster than it can keep up. Even when I hide the preview. (I think maybe it's generating the preview even when it's hidden?)
Anyway, not sure if anyone else (future self included) can reproduce this, but it's kinda hard to write anything when the editor doesn't keep up with you. ;)
Sorry to be so grumpy and complain about everything, but it's my honest opinion! Overall, I think Discourse is good and I especially love that I can write my posts in Markdown. Keep up the good work!