You hit the nail on the head: notifications and quote reposts.

Would also be cool to have the suite of command icons (reply, repost, zap etc) on the main feed column, even if it breaks out directly into the second column -- one less click to reply. And I feel like we should be able to zap or repost in the main column without having to break open the tree.

These are preferences though rather than raw functionality so depends on if that fits your vision!