It's nice to have a well-produced patch trailer again. 4.3's dinky montage and 5.1's utter lack of a trailer were a bit disappointing. I realize these things aren't all that important in the long run, but it's little things like this that can drum up excitement for the game.
I get the impression that Blizzard has put a lot of effort into making the Throne of Thunder an epic raid. We haven't really had a huge raid since Icecrown Citadel, and I've always found those to be more exciting than the smaller, broken up raid tiers.
Aesthetically the Throne of Thunder looks to be quite cool as well. We get plenty of Troll stuff (armored dinosaurs!) but it's not overwhelming, so we also get to experience some cool Mogu stuff.
While I would certainly like to see new 5-man content, I get the impression that Blizzard is de-emphasizing them as a progression path, other than a source of Valor.
Justice Points are also fading into irrelevancy, with old Valor gear remaining purchasable for Valor, albeit at a discounted price.
We have three new factions, though per-character you'll only see two, as two of them are faction-bound.
The Shado-Pan Assault:
This faction is the raid faction, and will be the new source for Valor gear. You will be able to earn up to Friendly rep simply by clearing trash in the raid, but in order to earn beyond that, you need to down bosses. Luckily, beating them in LFR will also count toward your rep, though you only get one reputation boost per boss per week, meaning you can't run LFR and Normal to double your rep. There will be some items to purchase at Neutral, though, so even if you get off to a slow start, you'll be able to spend your valor.
The Kirin Tor Offensive:
Jaina has mobilized the Kirin Tor and reestablished the organization as a strictly Alliance faction. Alliance players will fight alongside Jaina as they take the isle of the Thunder King, fighting the Zandalari and Mogu, as well as the Horde forces there, the Sunreaver Onslaught.
The Sunreaver Onslaught:
Lor'Themar Theron has taken direct control of the Sunreavers after their expulsion from Dalaran. Horde players will aid the Sunreavers in fighting the forces of the Thunder King while also grappling with the Kirin Tor Offensive.
Both faction-specific reputations will give you the option of PvE-oriented quests, sending you against the Zandalari and Mogu, or PvP-oriented quests, which have you fighting the other side. As I understand, you make the choice at the beginning of the day, and thus PvPers can stick to PvP and PvEers can stick to PvE.
Wrathion continues his chain, sending you into the Throne of Thunder to eventually put together the pieces for a Legendary Meta Gem, and an extra socket to put it in.
Warlocks will finally get their Green Fire quest chain. The chain begins, I believe, with a drop (though I do not know off of whom) and leads them eventually to a separately instanced Black Temple. After completing the quest, Warlocks can switch the color of their fire spells back and forth by talking to an NPC (in case they aren't crazy about the new look.) Hopefully this will herald in a new era of epic class quests.
Transmogrification restrictions will be eased considerably: Main hand and one-hand weapons are now inter-transmogrifiable, and any weapons that share an animation (such as two handed swords and two handed axes) can also be transmogged to one another. Vagaries of Time, here I come!
All in all, it looks to be quite the exciting patch. I expect we're going to spend the better part of 2013 with this one, perhaps with a smaller, 5.1-style patch a few months down the line (perhaps adding 5-mans?)
Of course, we don't know how many tiers we'll have this expansion, but if it holds to the pattern most expansions have, we'll likely see the Siege of Orgrimmar near the end of this year. And we can expect to hear about the next expansion at Blizzcon this year.