After a few more Zul'Gurub runs, I've revised estimates of the trash's overall difficulty far downward. However, anticipate a lot of mistakes if you're going to pug this content when patch 4.1 hits, and if you're healing, be prepared for some of these mistakes to be fairly deadly.
Click the cauldrons along the green foggy pathway once the associated NPC dies. The buff you'll get from them reduces nature damage by 90% and will keep you from being stomped by AoE damage and special mob abilities. The buff only lasts for about 20 to 25 seconds, so you'll need to continually rebuff yourself.
Before you get the cauldron buffs, the associated NPC will randomly select targets for about 6-7K damage per second attack. It's healable, but not fun. Once you've got the cauldron buff, the damage is negligible.
The two most important mob abilities -- and things you'll see in the Venoxis encounter -- are as follows:
A green line will sprout between two random players in the group and do damage each second to both them and anyone else they're around. Run away from the person to whom you're linked, because staying on top of him hugely
increases the damage. If you're tanking, be very careful if melee DPS gets linked to you or other melee and doesn't run away. That's actually a good opportunity to blow Stampeding Roar
to get players out quickly. If you're healing, holler at the group to run like hell.
Breath of Hethiss
You'll see this fun little ability trotted out by the two snake adds directly before pulling Venoxis. It's a frontal cone AoE that does enormous damage (to the tune of 26,000-27,000 per second) with both mobs often performing it at once. Pull the adds back to the nature resistance cauldron outside Venoxis' enclosure, and do not forget to rebuff yourself continually. The first sign that your buff's dropped will usually be death.
High Priest Venoxis ("snake boss")
The one thing you have to know about this fight: If it's green, it's mean. In addition to Toxic Link and Breath of Hethiss, Venoxis will also put a maze of poison lines on the ground and spend portions of the fight channeling three poison beams that will chase people. However, hurling all this poison at people gets tiring and he'll periodically enter an exhausted state where he takes enormous damage.
If you're tanking Avoid green stuff on the ground, watch out for Toxic Link, interrupt his channeled attack (it looks like a green version of the discipline priest's Penance), and be prepared to pop some cooldowns if your healer's on the run. However, your most important responsibility here to is to keep Venoxis turned away from the group and to run the hell out of the Breath of Hethiss attack, which is still a frontal cone AoE. However, here it's a frontal cone AoE without the 90% nature damage reduction buff from a cauldron. This can get tricky given the poison maze on the ground, so have a workable escape route before it happens. If you've got a choice between running over a poison line on the ground or swallowing a Breath tick, take the former. Also, turning Venoxis into the rest of the group while he's channeling the Breath is not the best idea ever.
If you're healing The opportunities for sudden damage on this fight are plentiful, especially if people aren't smart about Toxic Link. Efflorescence is generally of little use because people have to spread out and move a lot, but you'll probably be abusing Swiftmend pretty liberally regardless. The most dangerous portion of the fight with respect to group healing will occur as three beams chase players around the maze of poison Venoxis has created. If it comes down to swallowing a beam or swallowing damage from going over a poison line, encourage people to do the latter; crossing a poison line is only about 14,000 damage, but taking a beam is more than twice that. This is also a good point to pop Tree Form if you're being chased yourself.
Avoid large rolling rocks (this is generally good policy in real life as well). They'll toss you back with about 30,000 damage, which I totally took on purpose for the sake of this article and Science™ (right). This is problematic if you're healing and get belted out of range of an oblivious tank, so don't be
Trash is otherwise unremarkable, and you do not need the associated cauldron buff for Mandokir (or at least, you don't on the current PTR build). The cauldron was bugged earlier such that you could two- or three-shot Mandokir with the buff active, so it's only been relatively recently that the Mandokir encounter's mechanics have been clear.
Bloodlord Mandokir ("raptor boss")
Fans of the original Zul'Gurub will be pleased to hear that he still levels up with a "Ding!" (and reply of "Grats!") when he kills a player. This is actually a standard part of the encounter, so don't freak out if you're healing. Ghostly troll priests will resurrect fallen players, but you'll only get 8 of them, and Mandokir's "leveling up" is a built-in DPS check. With each level he gains, his damage increases, so ... yeah. The general idea is to kill him before he's capable of two-shotting your tank.
If you do nothing else on this encounter: Kill the raptor. Kill the raptor. And then kill the raptor some more. Why? The raptor kills the priests that resurrect players. Run out of priests, and you'll start running out of players.
If you're tanking Mandokir will start the fight mounted on Ohgan, his raptor. Tank him in the center of the enclosure to start. Before long, Mandokir will hop off and the raptor will become a separate mob. Ohgan can't be tanked, but you can help put some extra damage on him and keep Infected Wounds up to slow him down. Assuming your DPS is paying attention, the raptor should die well before he reaches and attacks a priest. When Ohgan dies, tank Mandokir on top of his corpse so you can nab him each time he's resurrected. You're immune to Mandokir's one-shot random kill, but your healer is not -- and Mandokir's damage will keep ratcheting up as the fight goes on. Keep your cooldowns for the latter half of the fight. Otherwise, avoid Mandokir's Devastating Slam attack. Where it's going will be telegraphed on the ground in a very obvious fashion with lots of dust and a red glow.
If you're healing You are not immune to Mandokir's one-shot random kill attack, so be prepared to blow a cooldown or two if you die at an inconvenient time. However, this is definitely an easier fight to heal than Venoxis if people are decent about staying out of spikes and your DPS is good. You will not need to heal or battle-rez anyone who gets stomped by Mandokir's one-shots, so just ignore the random deaths and stay out of Devastating Slam.