After a guild kills Gruul, there are three or four possible choices for the next boss to focus on: Magtheridon, Hydross/Lurker in Serpentshrine Cavern, and Void Reaver in Tempest Keep. Of these bosses, Void Reaver is the easiest, and Magtheridon is probably the hardest. Given that Void Reaver drops T5 shoulders, a lot of guilds choose to tackle him after Gruul. I believe that this is a mistake, which ends up hurting the guild progression in the long run.
Casual raiding guilds, especially those on a limited schedule, need to organize their schedule for progression. There are three competing constraints that a raiding guild needs to follow:
- Maximize the time spent on Progression instances.
- Minimize the time spent on Farming instances.
- Maximize the number of epics gained from the Farming instances.
You need to spend time on boss fights to learn them. The more time you spend on a fight, the better your progess is. But you still need to farm a beaten instance for gear in order to improve your raid. Spend too much time farming, and you won't get the experience on fights that you need to beat them. Spend too little time farming, and your raid won't improve gear-wise. (Not to mention that your raid will get discouraged from the lack of loot.)
The key to sustainable progression lies in how you manage your farming time.
So, keeping this in mind, let's look at the three options again. What happens after you put the target on farm?
If you kill Void Reaver or Lurker/Hydross first, your farming instance is Gruul's Lair. You'll get about 7 epics for your farm night, and you'll probably have to add Karazhan runs to supplement the extra gear. Or you'll end up working on SSC, but farming Gruul's Lair and Void Reaver, which takes a fair bit of time and nets 10 epics.
On the other hand, if you tackle Magtheridon first, it may take a bit longer to learn the fight, but once he's down you have a farm night of Gruul's Lair + Magtheridon, which is very fast and gives 12 epics. This allows you to give the maximum amount of time to the next raid instance.
In fact, I believe that a guild should ignore Void Reaver until SSC is complete and becomes your farm raid. Even though you can kill him earlier, going after Void Reaver earlier is non-optimal for the three constraints listed above.
In my opinion, raid progression should go in the following order:
- High King Maulgar/Gruul/Magtheridon
- Serpentshrine Cavern
- Tempest Keep
- Mount Hyjal
- Black Temple
You should concentrate on one instance, and farm only the instance behind it. As you move down, you drop the oldest instance in favor of farming the next instance. For example, once you've killed Magtheridon, move into Serpentshrine Cavern and drop Karazhan. Once you've killed Lady Vashj, move into Tempest Keep and drop High King Maulgar/Gruul/Magtheridon.
Note that this list is not organized in order of boss difficulty. The objective here is not to cherry pick bosses as you are able to, but to optimize for the three constraints. Maximize progression time. Minimize farming time. Maximize loot from farming.
I believe that is the best recipe for long term success.