One thing we now know for sure is that Molten Core will become accessible on November 21 (a week after the launch of Warlords itself) and last until January 6th, 2015. This gives you a little more than a month to get to level 100 and run the revamped Molten Core. It's a little worrying to me that you'll have to hit 100 and run this content (remember, it's a 40 person LFR) during the holiday season, but that's what we are currently being told. Also, thanks to the folks at Wowhead we also know that there are four helmets you can get when you defeat Ragnaros - an updated Crown of Destruction and plate, leather and cloth versions. In addition, Ragnaros drops the Core Hound Chain as well.
The new battleground will have a 90-99 and 100 version, and winning either version of the Southshore/Tarren Mill battleground will reward you with the Tarren Mill Terror title for Alliance players and the Southshore Slayer title for the Horde. Rated PvP play will begin on December 2nd, and the first raids will begin opening at that time with Raid Finder and Mythic difficulty opening the next week. A full schedule for the raid releases is incoming.
So that's how things stand so far. Looks like you'll want to get to 100 as quickly as possible if you're looking forward to that Molten Core LFR or the Southshore/Tarren Mill deathmatch.