I think that Blizzard made an acceptable decision to release raids as both ten and twenty five man versions.
Kara & ZA were a huge success in their eyes. Both were small enough for pugs to run with a strong organizer. Kara was entry level raiding that was challenging but manageable for players who ran heroics. ZA showed that ten mans could come in multiple difficulties. Therefore, they decided to continue making ten man instances that kept getting harder.
I was originally a little angry about this idea, as I felt that seeing the higher content was a reward for conquering hard encounters / progressing. But since then, considering the next point, I realize that higher end twenty-five man raiding guilds really won't have tons of general-chat company in Uludar / Icecrown / ect.
I think that 10-Mans of higher tiers will be less popular, and ultimately fail to be the Holy Grail for "Casual Players".
There were many people who, as part of small groups, beat ZA in only Heroic & 10-Man gear. These people have a ton to look forward to in Wrath. There were far, far more who screamed that it was too hard, tuned for T5/T6 gear levels, and went back to badge farming in Kara.
Naxx-10, currently, is easy. I'm not saying this from the "I roll in there with 9 other T7.5 geared players and train the entire wing's bosses together and AoE them down" stance. I'm attempting to say it from the "have raided in all three expansions, and am looking at gear requirements + encounter complexity" stance. I've gone in there with a wide mix of people, and even nights when its mostly green/blue geared players with no idea about the bosses, we can still beat the encounter with 1-2 tries. It sort of reminds me of UBRS, except that it drops epics.
I think that for a large majority of the "casual", "unenchanted greens are good enough, boss strat illiterate, lolwut's a theory craft" WoW player base, Naxx-10 is the end of the line, just as Kara was. If T8 is half as hard as I'm hoping for, the effort required will be enough to keep these players at bay - only now, they can't claim raid size as the barrier to higher content.
There will be guilds/players that go all the way only raiding ten man content, but they are a small minority. Most of these players that have the drive to conquer hard content get themselves into 25-Man raiding guilds. Only people whom have some specific preference / constraint that limits them to ten mans, and is a stronger pull then the lure of more "epic" feeling encounters and better loot, will end up in a "hardcore" ten man guild.
For the people that go this distance as a ten man guild: /salute.
For the people who can't: Do you prefer it to be called Kararamas or Naxxazan?