there will be guilds/people ready to level up 2 - 3 toons at once,and by the time the content will be released they are going to be ready to hit that naxx multiple times with multiple raid compositions to get those realm firsts,expect naxx to be downed in the first day since there is such a long time to gather enough resources for 80 gear to down naxx.
it might be true,maybe your guild will hold strong i know nothing,but we're talking about 4 months until we can see the first wotlk content release,expect people to leave,some of them might move on or find something else to do,don't expect all of them to hold to this game to the end like fanatics;i was once in a guild which i've been raiding with for 2months and i had to leave the game for a couple of months ,and when i came back there was almost no one who i used to know,only new people.