I raided Naxx with my main guild I was with through out all of Vanilla. We started and did the typical Raz and Anub fights but were stuck on progressing any further. Had lots of attendance issues, raiders not performing 100% etc.
I then joined a guild that was stuck on 4 horsemen. That fight was absolutely insane! You needed roughly, what... 8 tanks. Guilds that made it this far know that you sometimes had to bring other tanks in from other guilds. We never got them down before TBC was released.
However, since I changed guilds I missed out on a lot of fights in between 4 horsemen and raz/anub, so I never got the FULL experience. I do know it was difficult as all hell though.