I think the biggest question regarding this change is how exactly will WoW pull off making 10 man = 25 man difficulty?
I have been in situations where people dying have made things better. When Player D, with bad situational awareness, was better off face-planted on the floor instead of blowing up a bomb melee range. When 3 dps unable to stay out of frost patches died and the tank and healer finished off the boss because they healer no longer had to waste their mana healing stupid. Where having more room to spread out and not chain some horrific damage to 4 other people was beneficial.
In other words, I've always leaned towards 25-mans being more difficult than 10 simply because of logistics.
How will they counter this?
Albion island beginner guide
20 hours ago