Run Naxx on day one and you get 200 level gear, and whatever the 25 is, doesn't matter. When Ulduar comes it, it will give Ulduar level gear, but the boss damage and health will scale up as well.
Progression as a concept of "do X raid then Y raid then Z raid" would be essentially gone. You hit 80 and every raid is the top raid. Which one do you want to do?
I see little loss, only gain. Progression is already dead. We go from randoms to ICC. Some do ToC, but Ulduar and Naxx are entirely gone, along with EoE and dragonland.
There's still argument that the very last raid should be higher than the rest. Fine. Then put 25-man drops for previous raids at the 251 level, putting them on par with 10-man, but inferior to hardmodes and loot from Arthas. That would still leave a bit of progression without making older raids completely obsolete.
We'd have a wide variety of choices and could focus on those raids which we enjoy or haven't farmed a hundred times.
Why is it hard playing new MMOs?
2 hours ago