This is partly true, there's always a point around this time in an expansion where the boards start to get a bit stagnant. It'll probably pick up a little when WoD is released, however the trend over the last expansions has seen the period where forums are active after the expansion release (or even new content/seasons) dwindling shorter and shorter each time. This is probably not helped by 'new' content hardly feeling that new after 10 years of it, and rewards not really feeling that rewarding anymore.
Like somebody above said, after WotLK something happened (whether it be the game being tailored to 'casuals' or the RBG introduction, or ridiculous rating exploiting, gear becoming the main factor in your success meaning that playing alts is not even fun - whatever) that killed (or at least brutally maimed) WoW for a lot of people.
I'm in the same boat as you OP, in that I occasionally come back to have a look here from time to time and wonder whether I might re-sub, just to fill a month or two in between work and uni or whatever, where I have time, but it's never very encouraging.
Not that you'd get a team anyway as gearing up is, by all accounts, ridiculous, and it's the same 10 players occupying the top 50 spots on the ladder nowadays. There is no dipping in and out of WoW now, you either keep up with the curve throughout the xpac, or you wait for the next one.
Makes me v sad.
Edit: Interestingly, it was during WotLK that this forum (in terms of threads with largest amounts of views/replies) was most active. Though I definitely seem to remember it being the most useful (and having the least amount of trolling) in BC.