The first time when I entered and played through this instance, it was not only normal difficulty but also had many of the mechanics hardly in use. This was true particularly with the boss fights, which lead me personally without a sense of satisfaction. The instance is well designed, atmospheric and just beautiful to look at. The same is true for the Forge of Souls.
However, I a much better sense of the mechanics this time around and I even had the sense to grab a couple of screenshots. The nice thing about it is that, unlike so many other instances, Pit of Saron is not linear. You can go about killing the first two bosses whichever you want first, as well as free 20 slaves from wherever pleases you.
At the moment there are more slaves in the pit than you need to be able to save, so you are free to take your pick about your route and targets. The first boss, Forgemaster Garfrost is simple enough. As his health drops, he will stun you party and jump between his forges where he takes out a new weapon every time.
The only tricky bit of the fight are the adds around Garfrost's forge. Neither the boss or the adds can be pulled separately, so you have to quickly burn the adds down so you are not overwhelmed. Other than that the fight is still extremely simple; just tank and spank him.
Next up is the leper gnome on his private hunchback minion; Krick and Ick. I was happily surprised, particularly by this encounter because the first time around it was almost like fighting trash. Krick will drop poison buddles much like Grobbulus and needs to be kited. And by kiting I really mean it because the little twat is obsessed by his buddles.
He will also occasionally chase on your party members, when he uses an ability called Pursuit. I am unsure what else they can do at that point, except just run like the wind. It was not an issue for us, for we were insanely over geared for the encounter, but I can see it particularly nasty for low-gear level 80s.
Additionally, he will occasionally go through cycles of using poison nova and small arcane bombs around his immediately surrounding. He does not need to be tank during the bombs so both the tank and melee can, and should, freely circle around him and avoid the purple domes. The poison nova is a fairly high damage, so like during overloads on both the Iron Council and Mimiron, the party should get out of range.
The next part is definitely my favorite, as you must run through a gauntlet of undead, including a tunnel that is currently collapsing. Remember Hodir and the falling ice? Same deal, except you will tank trash and must keep moving at all times. Any tank who loves mobility and AOE tanking, this is the place for you.
The last boss has not changed much since my last visit. Tank him approx. where he lands but mindful of the frostbolts that his mount drops on your party. They fall fairly randomly and easily freeze you where you are standing. The good news is that the person who gets hit is marked by a hunter's mark, so you always know who it is.
Other abilities that he has include Forceful Smash and Overlord's Brand. The first is just a very strong hit that your tank has to soak. It hits for about 15k on heroic, so your party healer can keep the tank topped up, it should be a small problem but can be an issue if the healer gets frozen for instance.
If healing is scarce, the tank can always use a cooldown to help out. The brand is more annoying, as when it hits a dps player, all the damage produced by that person is diverted to the tank. If the target is a healer, all healing is directed to the Tyrannus.
All and all, the instance is only getting better as the PTR goes on and I think we can see the entirety come together as a very nice and coherent dungeon. If you are interested to see some video feed of the dungeon, I came across a good quality look at youtube.