My suspicion (and this is purely speculative) is that GW priced some people out as well. The only reason I'm here presently is that I'm not trying to build up my new army in a reasonable period of time anymore, and am rather just going for a squad a month or so. That's not a huge deal for me, since because of real life time limitations I wouldn't get to play a ton these days regardless of if I had a full army or not. What's left to me, really, is enjoying a slow build up and paint process (and if that interests you, I'd encourage you to check out the challenge thread over in the painting forum. Lots of activity in there, as well as cool pictures and tips, and it's not to late to get in on the ground floor for 2014). I suspect that some people though, don't really enjoy the painting and build up aspect all that much. So when prices got pushed beyond a certain point, and GW started making a point of pushing out several $80 kits, they had issues.
I remember back in the day when I first got exposed to this and started up, my parents weren't fans of the hobby at all and I was still a little kid. So I had to save up little bits of money here and there to sporadically put together my first army (it helped that cardboard dreadnoughts and griffons were allowed and included!). As I grew older and had access to more personal freedom and money, I ended up giving greater sums of it to GW. In the present day, I'm pretty sure that no kid would be able to very successfuly put an army together by saving up little odds and ends like that. I see that as a lot of missed opportunities by GW to grow the hobby, in favor of more immediate returns from existing customers. But when existing customers slowly take breaks or leave the game, and you stop bringing in as many new ones, it's natural that forums slow down a bit.
So yeah. The forum is a bit quieter than it was. I hear that's sort of how it is across the board with forums these days, as the young kids play with their tweets and hashtags (I'm not that old... I don't think.... but sometimes I feel it when I realize how much of this social media stuff I don't use or understand).
Still, there's stuff going on and a solid bunch of people seem to have stuck around.