Eternal September is named the way it is because in the past online communities only had to deal with influx when students got to college, but now they have to deal with a constant influx. Given this, it seems obvious to me how you go about countering Eternal September: remove the eternal.
This could be done in a few ways, but the core idea is having a period in which registration has a barrier for most of the year and periods in which the barrier is removed for the sake of growth.
EDIT:
There should be the ability to be more granular in posting. More topic options, subscribability etc.
I've said it before - and I'll keep saying it - take the best aspects of Reddit and apply them here.
Why can I easily see top, new, threads and comments - but I have to go to my profile to see my submissions? I can't see if there have been replies to my posts etc.
Rather than making it harder to post - lets make what we already have better to use.