We only found out about haki during the time Luffy and others were saving Robin. We saw haki in the very first chapter and the first anime episode, but didn’t know what it was.
Now Mihawk explained something new and that is that you can cover your sword in haki and not only your body.
Mihawk was amazed the first time he met Zoro. He believed that there is a lot of potential in Zoro, because he was ready to protect his friends even though it was obvious that he had no chance.
Mihawk was the one who later on taught Zoro how to fight and is now waiting for him to surpass him.