There's been a lot of information about 4.3 coming out lately... some good, some not-so-good.
Look For Raid seems like a sure thing, and it will feature a so-called "easy mode" for the raid. Easy mode will provide weaker gear, but there's been no mention whether or not this will be similar to the difference between normal and heroic (i.e. same gear, different ilevels).
There will be 3 new 5-man dungeons. The first is where you will go into the future to see a post-apocalyptic Azeroth (Dragonblight actually). You will fight 2 of 4 old leaders that are some sort of spirits in agony (Sylvanas, Blaine, Jaina or Tyrande). Then you fight some dragon from the Infinite Dragonflight that's blocking Nozdormu's ability to send you back in time.
The second is where you go back in time with Thrall (now that you can) to retrieve the Dragon Soul. For the non-lore nerds out there, the Dragon Soul was created as a way to harness the powers of all the dragon aspects, and said aspects were tricked by Deathwing (then Naltharian) to giving up their power for malevolent purposes. Supposedly, we will be using the Demon Soul to kill Deathwing.
The third dungeon involves us going to Wyrmrest to confront Deathwing himself.
There will be a new legendary item, or in this case... items. The new legendary is confirmed to be a rogue dagger
set, which should work for combat rogues too. This will involve the black dragon that we saved in Badlands, and said dragon will provide you the daggers for performing nefarious deeds to help him. Apparently, not being very nice runs in the family.
The new raid will feature 8 bosses, which the last two are Deathwing oriented. The 7th will involve us on Deathwing's back as he races to the Maelstrom. He will supposedly be trying to shake us off during this time, so who knows what crazy mechanics this will involve. The 8th will involve fighting Deathwing at the Maelstrom and we will hop around different floating platforms and it includes help from various dragon aspects.
The raid will also feature a slight buff to help out melee classes.
Epic gems will drop in the new raid, but only on normal and heroic. Each player that defeats a boss will receive a sort of "gem cache" which
rarely contains an epic gem. They said it will take awhile to replace all of your gems.
Corrupted Ashbringer will not be available from vendors for Transmogriphying (

), but the old PVP gear will be.
EDIT:
Tier 13 will
not be available for valor points. Assuming it will drop from the raid only (and BH).