The Diablo 3 panel started right after the opening ceremony and two major announcements were made:
The fifth (and final) character class was revealed to be the Demon Hunter. The introduction and gameplay videos both showed only the female version. The character seems to specialize in ranged combat using crossbows, shadow magic and traps. Interesting details is the Demon Hunter used dual-crossbows in the introduction video Remains to be seen whether the class will be able to dual-wield crossbows in the game. You can find more information about the Demon Hunter on the character page of the official Diablo 3 website.
The other major Diablo 3 announcement was related to PvP. It was revealed that there will be arena levels in the game, all of them unique and with different types of terrain and cover. The PvP matches can be class-based or team-based and it appears the loser will drop an ear, just like in the predecessor game. Blizzard are promising more news in the future regarding the types of matches that they are going to make available. Read more about PvP battles in Diablo 3 here.
Blizzcon 2010 is continuing today and if more news about Diablo 3 are revealed, I'll make them available to you here