Maybe it was just my experience, but I felt like in vanilla WoW your guild mattered more. I don't recall any of our members ever running MC, BWL, AQ40, or Naxx with other guilds/pugs unless they intended to leave which was uncommon, even when we were struggling with Huhuran. In modern WoW it seems much more commonplace to immediately dump your guild the week that progression stalls.
I wonder why they don't add some non-performance roles in raids, like operating seige equipment, doing a puzzle, or more vehicle fights like Ulduar where some vehicles are really easy to perform in. Instead of your entire raid having to be really good, you could put the folks of lesser ability into an easier role where it's hard for them to mess up and they can still contribute. In an epic fight with a dragon certainly lots of people might be loading trebuchets or firing arrows from a distance, not necessarily dodging close range swipes and fire. I remember EQ2 had crafting dungeons which I thought was interesting. Anyone that played Ad&d with a decent DM could utilize a lot of non-combat abilities to be successful.
I think if WoW could layer raids with some interesting roles for folks who are not as great with the mechanics, it might help guilds stay together a little easier instead of falling apart so quickly.
