Well...I'm putting my vote in for an "Advanced" forum. I like the idea, and I remember it on the older forums off and on. I think that now more than ever it is a good idea, as Yoyo is attempting to target "higher-end" customers along with the "hobbyists" and I'm sure plenty of the hopeful new customers would want somewhere to discuss advanced topics.
Honestly, I don't think traffic should be a deciding factor. Is there an actual cost to having a sub-section that doesn't have tons of posts coming in? I understand that there is an issue moderating it, people hijacking threads, or simply putting non-advanced topics there. But it seems like people are afraid of having forum areas where there is little to no traffic, and I'm not exactly sure why that is(Maybe someone can enlighten me). On the other hand, having an advanced section with a select few advanced topics, as in there are few enough that they don't get shot of the page within a few days...I think someone investigating will believe how powerful GMS actually is. Right now, the forum doesn't really show much of that, at least not all in one place. I see it the same way as a 3d sub-forum. The random person looking at the GMC right now doesn't see much as far as 3d or advanced topics, because though said topics DO exist, they are quite spread out, and get pushed of the front page with all the "novice" questions coming in. However, a nice 3d subsection means that there is at least a slim amount of 3d capability in the engine, and in fact if someone wants to know what people are working on in 3d, and possibly what issues they are running into, what workarounds they have to do, etc... a forum could be the best place for that research, as that is where questions, issues, and workarounds would actively be discussed.
The same applies to the advanced topic forum. Things like destructible terrain could be discussed there(and have in the past I'm sure many of us remember). Looking once again from the point of view of a potential "high-end" customer, as it stands now, I don't see much really advanced usage of the software here on the forums. I would never guess at the kinds of advanced things it can do. But if I were to run across an advanced forum, with topics like destructible terrain, sophisticated AI, and other things that would fit, it would queue me in to the fact that GMS is much more capable than what many people would think. Look at it from that point of view.
Basically, I see it as something that benefits everybody in both short and long term. We get somewhere to discuss our topics, yoyo (in theory) could end up with more customers, especially if after there are some nice advanced topics, they could link to the subforum somewhere on the website in the features, like a "see what it can do" kind of thing. And then us users hopefully get benefit there as well in that the more customers Yoyo gets, the better they can make the software. Sure, some of this is theory, long-shot, whatever, but really, if a way to work around the moderating issues can be found, either by simply moving topics out, deleting posts that don't fit, or if you somehow only allowed the "elite" posters to post(but you have to allow everybody to view at the least), then I don't see what we can lose by having said sub-forum. And lack of traffic isn't really a valid excuse in my opinion, nor the possibility of "clutter" in the table of contents/sub-forums. You can always have sub-forums that don't clutter things up on the primary pages.