Don't know if this is the right place for suggestions, but I have a few that were immediately obvious. (Please let me know if there's somewhere more appropriate to post these!)
I started a new thread in a quiet sub where the last post was 2 months ago. However, my thread didn't show up at the top (when using the default "hot" view). That would be understandable for busy subs but if the latest post is months old its "hotness" should have decayed enough that a new post goes to the top.
I also got an error when submitting the thread, although it did go through. I ended up double posting because of that. (Also it wasn't totally clear it was an error at first as the page itself didn't say "error", only the title on the browser tab.)
The dropdown menus suffer from the "diagonal problem" where they disappear the instant the mouse moves out of the hitbox. For starting a new thread, I hover over the + then move diagonally down and left towards "add new thread" but the menu disappears.
Someone else mentioned it, but comment folding is a must.
Don't know if this is the right place for suggestions, but I have a few that were immediately obvious. (Please let me know if there's somewhere more appropriate to post these!)