Keo here has a point, kinda.
I can't imagine myself and my guild progressing on 10m. How can we even call this progress? I could yell on trade and find 9 random ppl and we could kill things at 10m and end up with same gear that you can get with 25m. The only difference is. You will get it slower than on 25m. Thats destroying the whole PvE idea of this game. Thats even destroying the idea of "The Guild" since you won't need it to actually go into instance and kill something. To make it more ironic. All the changes regarding Guild in Cata will be kinda worthless since having a guild will not be needed anyway. It's nice to have in one place ppl with which wana raid. But 20m guilds are like what? Blizzard made top 20ppl that will contribute to the guild. So in Cata, everyone in guild will since the guilds wont be bigger than 20m I suppose. There will be nothing in 25m instances that will attract ppl to do them since it's all the same on 10min. The image of raid for me is lost. Once you needed 40 people to kill Ragnaros, now You will need 10ppl to defeat Deathwing.
I do not like this change, at all.
ps. sorry for bad english