Okay, I had a listen to all of the panel. The main points of interest to me were what they were working on.
Jens, Dinnerbone are adding features for the 1.4 update. Dinnerbone said it was going to be a nice decoration update but they are aiming for release near Halloween, so it should have a scary side. hence he named it the "Pretty Scary" update.
Grum is working with some people from the community on redesigning the client completely. He commented that it was originally design to cope with some old pcs but the new ones lost out as a result. They are trying to improve on that as far as performance, as well as expand the scope of what mods will be able to work with in the client in the future, e.g. adding new textures, etc. The Launcher is also being looked at so that it can be updated without having to manually download a new version. Interfacing with Mojang better for getting updates and snapshots is also under investigation.
EvilSeph is working on the bug reporting system. They are going to run a "testrun" first related to the Mobile version of Minecraft so people can report its bugs. Then they will move to the API where ideas for additions will also be able to be submitted. In the long term, they will set up a system for the whole of the PC Minecraft game.
Nothing was really said on the Adventure side of things except that they have thought about adding stuff like Factions but found it was too complex for the vanilla Minecraft. The Beacon that Jens is developing is the outcome of that investigation.
Oh, and the coming Wither boss will be "summoned" through some ritualistic action by players using rare mobs drops, like the heads of the creatures. Nothing was said about what you win when you kill the Wither.