My personal thoughts (I am not a lawyer, medical doctor or whatever):
Questions with obvious or stated nefarious intent
Such as clandestine manufacture of explosives, chemical weapons or generally illicit substances should probably be prohibited. This means syntheses that eschew scheduled precursors, jury-rigging apparatuses for these syntheses and questions about detectability and deniability. If you have a legitimate need and legal ability to do any of this, you probably don't need to ask questions on SE.
Questions about working with scheduled substances
There are many extremely useful and basically benign substances on schedules and lists because they might be popular for that one step in making meth. This does not mean questions about such substances are bad, even without context. However a little context couldn't hurt these questions, especially if the questions are on the pseudoephedrine end of a list rather than the acetone end (acetone is a DEA List II precursor, weird!) and as such people should feel free to request clarification.
'Cool Chemist' questions
'Cool chemists' can be divided into two categories - the 'less cool' ask questions about dangerous or fishy chemistry out of purely idle interest, and the 'cooler' actually attempt it. A decision to moderate should be tempered by the academic interest of the question (say, a question about the reaction kinetics of an explosion vs. 'will TATP explode if I hit it with a hammer?') as well as the achievability of the proposal (there was an odd discussion on phys.se about whether a question on nuclear bomb implosion trigger design should be closed.)
Questions about working with intrinsically hazardous substances, reactions and practices
Should be potentially tagged or annotated in the body of the question as such by either the asker or a mod. If we can get custom tags or other annotations that are boldly coloured or have little DOT HAZMAT icons on them, that would be cool. At one point TeX.SE was debating the inclusion of 'worst-practice' and 'don't-do-this' metatags but decided against it as it represented a potentially socially caustic value judgement, however I think that in some circumstances such tags would work quite well.
Non-preparative questions about hazardous or scheduled substances
Questions about (for instance) why explosives explode and how toxins or illicit substances act on the body should be fine.