I am going to strenuously advise against using this format for resolving what should be considered on topic for this site.
I can appreciate the desire for decisive action and expediency, but it would seem untenable to tell someone their question is off topic because only the top {x}-voted subjects were deemed on topic, while {x-1} didn't make the cut. You are essentially looking to exclude large areas of interest by polling a relatively modest portion of an upstart, growing community. Polling is not a good substitute for discussion; this is not how you build a functional site.
You have a good site here. You started with a community that was very welcoming of questions about 3D printing in general. If you later discover that a particular subject poses a problem, then the community can decide if you should forgo those questions entirely. But that comes through thoughtful discussion in an open format — a conversation about why a particular subject poses a problem — not by a popularity contest where only the most-voted subjects get in.
Users can raise concerns about any topic they have here — that is what meta is for — but please resist the temptation for excluding content before it demonstrates a wide-spread problem in actual practice. It's a recipe for a very exclusionary site; one where everything is looked at through a lens of suspicion where everything is off-topic until proven otherwise. This is not the way to build a healthy, growing site.