While it would be nice if old worlds could generate the new block types, I can understand why Notch doesn't try to do that kind of thing. How would Emher and his friends feel when they log in after the 1.6 update to find long grass all over the pitch of their stadium?
Besides, it provides me with a new excuse to explore new regions of the world.
I finally did something that I wanted to do ever since the halloween update: I built an igloo!
I thought that the ice-windows were a nice touch. Now the game just needs penguins and I'm set!
According to his twitter, Notch didn't realise that there were still huge bugs with 1.6.4 so he proceeded with Mojang's gaming Friday today. When he got home and found out, he decided to go into the office tomorrow to work on a new patch.