Karazhan is a stepping stone for all guilds to the bigger raids that are 25 man. After Kara, you usually hit places like Gruul's Lair and Serpentshrine Caverns. Kara is a big milestone for guilds when they get to it. It's also where you'll be farming to get geared for the 'next level'. However, I've always seen Kara as a stop sign as well.
Being a self-termed 'semi-casual' player, I've thought about what it means to do 25 man raids. Can I commit the time? Will my guild be able to commit the time? Will we really be able to tackle them on a 'casual' basis? While most in my guild I think are optimistic, I'm the 'glass is half empty' guy on this one. I just don't see it happening. Kara has already proven to cause problems and some feel we are moving into 'hardcore' territory with it so anything beyond seems like it's too much.
To me, a casual guild basically stops at Kara... maybe Gruul's Lair, maybe SSC. It just seems to me that it will require too much time commitment per week to advance through those like other guilds. In a way, I don't think we even have to worry about it as the nature of our play style/time and the evolution of the game progresses, we likely will never hit the upper escholon raids before more new content comes out (like another expansion) and makes the current content outdated.
Zul'Aman is a prime example. I think by the time we're two-thirds of the way through Kara the next contect patch will be released. Blizzard is adding something for the more casual players with Zul'Aman. You're supposed to be able to tackle one wing of this new 10-man raid zone in about an hour, I believe. Either way, it'd be like taking Kara and chopping it up smaller. Also, there's no '1 week reset'... the instance resets each day. This means you don't even have to worry about Raid IDs and forming raids for the week. Just form your group and go - like any heroic 5 man.
So, what am I getting at? Well, with the recent announcement of Wrath of the Lich King, I think we can forget about 25-man content altogether. With ZA released next patch, I think by the time Heroes clears Kara and can 'farm' it and by the time we can clear ZA, the next expansion will be right around the corner. We may venture into the 25-mans to see it but I doubt we'll take it very seriously when another 10 levels and new content will make anything you get out of there ultimately useless.
I have mixed emotions about this. While I would like to see all content in the game, I'm not too broken up about struggling with 24 other people for hours in an instance wiping over and over again while people get frustrated about who's not geared enough, who's not doing their job right, and who didn't get to go on this run.
I was hoping with the announcement of the next expansion Blizzard will announce they were shifting focus to 10 man raids but it seems they are happy with TBC and the 25-man raid focus so the only thing I can see in the context of raiding in the expansion is to look for the next 'Karazhan'.
This game is a treadmill that resets your progress every time you think you're done.