One bad thing about doing 10 AND 25 man is how quickly boring the raid instance can become.
Except for Sartharion 3 Drakes 10 man, nothing so far in all of the 10-mans seems harder than 25s. The fights are essentially the same. As a result raiding both places is really raiding the same stuff twice.
I haven't done a 10man Ulduar since the first week and to be honest I enjoy my 25-man raids more because of it.
Now I'll admit I liked having a leg up on the fights that I had already done in 10-man before 25. However as a healer our excitement is pretty darn limited.
Here are the challenges healers face that make raiding fun imo:
1) Learning the basics of the fight
2) Being undergeared causes you to have to improvise how you play
3) Everyone else learning the basics of the fight and being undergeared as well
1) you're learning to not stand here, not cast there, move here, not move there. All of that is fun.
2) You're having to be more careful with how you heal. You may not have enough mana because of gear. This is actually fun to because you have to think a little more.
3) Everyone else is also learning and undergeared, but as a result we, as healers, have to do extra. This is where we shine. How many healers do you take on progression? How many healers do you take on farmed content?
Those 3 things only happen at the beginning of new raid instances. Soon enough everyone knows where to stand. You don't have to heal 5 noobs standing in the fire. You only have to heal 1 (or just let him die and be better off).
You eventually have great gear and you can spam one spell if you want, it heals enough and you have a big mana pool.ZzzzzzzZZzzzzZz
And finally everyone else is better and, sadly for healers, no longer need you as much. "You want me to respec laser chicken?"
So if my glory days as a healer in the terms of a raid cycle are fleeting, why would I want to rush them? Why do I want to run 10-mans and get geared up faster and basically do my PTRing on 10-mans so the 25s are just tweaking a bit and mimicking what I did before?
I know this isn't going to happen but to be honest it is what should happen - They should make a 10-man instances totally different than 25. Of course that goes back to 10-man fans complaining they don't get to see the same content as what is in 25. But to be honest those who prefer 25s could make the same complaint.
They risk burning out everybody even faster with the 2 instances being exactly the same.
What would you have prefered? Zul'Aman or Black Temple Lite?
34 minutes ago