Well,
...I am not, even remotely, one of those who posts a lot on this board, although I spend way too much time on it - it's one of the more commen entries in my history files both at home and work - and I feel I can comment a 'wee' bit on the amount of posts and how often people post.
I use to post a lot, mainly to yell at some wingnuts, on a rightwing news board I won't really mention - but it was fun to yank their collective chains from time to time, although I'm a right-leaned moderate - and when posting on a place that you like to be opinionated on, sometimes even a modicum of education, helps when your post count.
These boards are, in my opinion, a rariety in that people are, even when they're being a bit mean, pretty polite to one and other on here - even the so-called curmudgeons are nicer then a lot of places.
One thing though that seems to aid such massive post counts for those whom actually post a great deal is the fact that gamers, in general, have access to one, if not multiple, high speed internet options and, thusly, can rip through the boards in a short time and comment upon the various places they wish to add their voice.
Basically it's a message board about a hobby that, for many of us, is a significant portion of our life, so it's quite easy to talk about it in great depth and verbosity.