For me, the now-hidden formatting options was the biggest annoyance. I use quote/indent/lists a lot. Now I am slowly getting used to the new UI because I have no choice. I have new compose enabled on the @gmail account, although old compose is still active in Google Apps account somehow.

Also, as @tomkarlo suggested, use undo_send, that is a life-saver. I set it to max possible- 30 seconds.