The trash pulls were somewhat tightly packed on our way to the first boss, Forgemaster Garfrost. To be honest, it felt like we could have done either boss in order, Garfrost was just the first one we chose each time. There's a fun event involving Scourgelord Tyrannus where he animates a whole host of Alliance (or Horde, I assume, depending who you are) soldiers as they attempt to assist you, neatly explaining why Jaina doesn't just send in the troops. The answer? Because they'd get horribly killed and turned into zombies.
As an aside, my DK would like to buy the ability to use Strangulate on 20 people like Tyrannus.
Trash pulls are pretty easily AoE tanked at the level of gear we were at, but I bet a group in blues would want to use some CC on the two caster/two melee pulls. The big caster with his summoned skeletons seems single tankable, but beware: it's very very easy to end up pulling several packs at once. Especially if you have Alex "Look at my butt, giant zombies" Ziebart in your party backing into pull after pull. I think we pulled six straight packs at one point. The vrykul mobs mounted on dragons dismount and do an attack similar to the one in Black Temple, throwing their swords around to whirlwind independently, but we didn't seem to have much trouble running away from it.
Garfrost himself seems to come with several adds that cannot be pulled without aggroing him. You fight him on a large platform surrounded by several forges, and he has a knockback ability, a stun and an AoE debuff that stacks, increasing everyone's vulnerability to frost damage: you can clear this debuff by standing behind large saronite boulders he throws at members of the party. (As tank, you'll really only be able to do this when he punts you backwards and runs to one of the forges to equip a new kind of weapon.) In general it seems like an interesting fight with several elements to keep in mind, but both Matt and Tristan commented on how undertuned the AoE aura debuff seemed, and Alex pointed out that it didn't do any more damage on heroic than it had done on regular.
At present none of the mobs seem to have loot tables.
Continuing around the Pit of Saron itself (which really is a great big pit with a huge, crazy looking thing I did not get a good screenshot of, possibly one of the Soul Machines mentioned in the Blizzard page) we made our way to Krick and Ick, a leprous gnome servitor of the Lich King and his Master Blaster-esque plague spewer 'mount' for lack of a better term. This fight was pretty straightforward: they hurl plague patches onto the ground that you can't stand in, Krick activates bombs periodically that hit for AoE damage so you should run away from that, too, and... that was basically it, really. Know when to run away and you're good. (Again, part of this could be our ridiculous over-gearing for the encounter.)
Again there's a fun moment with this boss as Jaina Proudmoore grills Krick about Arthas and the Scourge, and then Scourgelord Tyrannus again Strangulates him from way the heck away. Seriously, did I just miss a rank of the spell?
You then rendezvous with Jaina and the various Alliance slaves who have been sort of freeing themselves as you slaughter your way through the place (the instance text makes it seem like you're freeing them, but I can't think of ever doing anything aside from killing trash mobs I was planning on killing anyway) and you are forced to run a gauntlet as Tyrannus commands his frostwyrm mount to collapse a tunnel on top of your heads while you try and make it to him. I had a blast careening from mob to mob here like a little gnome pinball, using Warbringer and Shield Block to generate just enough threat to pull the whole tunnel out before we got swamped by AoE damage from the falling ice and rocks. Again, in lesser gear I can see this being a PuG wiping grinder.
Finally comes Scourgelord Tyrannus. He dismounts his frostwyrm and fights you while his mount hovers overhead throwing frost bombs at random party members and masses of undead swarm your Alliance helpers: there's a definite DPS race feel here as the zombies keep spawning and your helpers go down one by one.
As you kill Scourgelord Tyrannus, several things happen. I'll let the above picture serve as a visual teaser: I don't want to spoil the final events of the dungeon, and besides which, there was a general sense that they weren't completed yet anyway.
All in all, I'd say that for a group without access to Tier 9, normal Pit of Saron will provide a reasonable challenge as it currently stands on the PTR. I expect heroic will be tuned upwards, as several mechanics felt unchanged from regular even taking our gear into account. All in all, it seems well scripted and entertaining, with several moments that will amuse, excite or surprise you. I can't wait to see the other two.