I get 10-player raiding to some degree -- I've been there myself! I've been in both ships at some points. However, I'm mostly excited because I think having one fixed size for cutting-edge content is going to produce better quality content. If X amount of time is spent balancing Heroic difficulty for both raid sizes right now, I feel like we will have better balance as a whole delivered for that level of raiding.
I like the Flex technology with respect to Normal/Heroic (which is basically equal to Flex and Normal respectively right now). The scaling technology is great and I thought this tier's Flex difficulty was a lot of fun. It basically felt like LFR for good players. Now we can apply that to the current difficulty of Normal mode to give 10-player guilds leeway to carry a bigger roster than they need to protect themselves from attendance issues while being forgiving to 25-player guilds who might be missing some people on a given week.
Sure, the competitive 10-player Heroic guilds are going to have to change or go home. Seems like it's necessary to achieve the end goal here though. Connected realms and all that is supposed to bring those lower-population servers together so they should be able to recruit and merge if they want to continue to be as successful. There's going to be a smaller overflow of 25-man raiders who might not have a 20-man roster spot (since most guilds carry a roster > 25 as it is) so really this problem should even itself out if guilds are willing to adapt.
I think this is a good thing for WoW.