Tiny UI tweak: If you move the subject line (Inbox / Discussion with Foo about Bar) below the navigation buttons (Inbox - Mark as unread - Share this conversation - 20 of 22 < >), then the navigation buttons stay in the same place instead of moving up and down depending on whether the subject line wraps.

This way, if you're clicking through lots of items, the mouse can stay in the same place and you can just keep clicking instead of having to reposition the mouse once you notice that clicks no longer move you forward. Less important once you get keyboard shortcuts in, but still handy.