However, what I think is really the wrong place to worry is mathematical questions that are just a bit too simple, as I can see little harm coming from them; but I can see why others see this differently and also for me this was a bit of a learning process/developpment.

For math.SE I do not know much. But different people have different motivations. I think you give one reason that likely applies to some, but I am quite sute this not all; I think some people just really enjoy this kind of teaching happening there and not here, or just learned about it first and see no reason to particpate in two sites. In any case, conversely, I for example do not forsee participating in a site like math.SE except on rare occassion. (This is not meant against the site; but I know my limitations.)

Just briefly, anything that I perceive as an honest mathemcatical question of an at least somewhat advanced nature ('graduate level') is fine with me.

Whether a certain type of question attracts similar ones. Yes, certainly. Based on observation and common sense. One can observe this on small time scales even.

1. Somebody asks on marijuana, and hours later, somebody else asks referencing this question on some less (potentially) controversial aspect of math and psycholgy.

2. Some said here they do not ask certain questions as they know they will be closed, so if they'd observe some similar ones that would not be closed, I assume they would then ask them.

3. Some people arrive here searching on the internet. We had some time ago a (controversial) question on name changes; it turned out OP found the site via searching, due to a somewhat similar question, so thought this was certainly on-topic here.

4. Somebody else explained here on meta that the first impression of MO was that it was mainly for soft-questions; this was the time of 'department closure', which by the way created in an immediate way a follow-up question ("laid off faculty" or soemthing like this).

Just a small selection.

.

^tl; dr

but the actual post itself caught my eye, so I'll change my response from a simple tl;dr to a much more appropriate

^tl;rd

(really disagree?)

My opinion is this:

MathOverflow is a resource that is (necessarily) shared by a quite large community with various different interest and ideas.

If in a large math department or just whatever large building there is a common room to be shared by everybody then the more viable strategy to avoid problems and conflicts is that those activities are allowed that do not disturb or alienate too many people. As opposed to a strategy, the analog of which OP suggests as far as I understand, where everything that some people find a cool idea is allowed and then needs to be tolerated by the rest (and this remains true if 'some' is a considerable number). Even more so if the intended activity of these some is outside the purpose for which the room exists in the first place. (This 'outside' does not apply to the recent example, which is ABC I guess, for this there were other reasons, which I won't reexplain here since I did so at (un)reasonable length already. But it does apply to a whole lot of question I think OP has in mind.)

To formulate it slightly more strongly than I mean it and considerably more strongly than I vote:

A question that is not a focused research level mathmatical question is theoretically off-topic on MO; however, it can happen, as any exception to the general rule, that such a question is still tolerate despite it being theoretically off-topic. But, as soon as somebody complains about it, it should be closed.

To get the idea better across why leaving everything open that some might like could be a problem, let me recall from memory, it is meanwhile deleted in case doubts arise to the veracity of my claim please some 10k+ person help me although I cannot be sure the comments to which I refer are still there, that we recently had a question on "Mathematics and marijuana" which was closed quickly (by some including me), which resulted in somebody belitteling or insulting me (depending on how one takes this, and not me personally but those that closed) by saying something to the extent that either we smoke a lot and do not want to get this to be known or are conservative small-minded people (this is not the exact formulation but the meaning was this and the choice of words was at least not friendlier). This comment came though not from a very frequent user of MO still from an occassional one that would be in the standard audience of MO (by academic background), as opposed to some random troll. And then some high rep MO user joined in saying that all this closing is really a big problem.

So, we ought to allow "Mathematics and marijuana" so that some are not unhappy.

Now, in the very end I would have less problem with this question, then with some others we actually have. But, still, if the criterion would be 'some find it interesting' a lot of things would happen, and I have no reason to assume that things like "Mathematics and marijuana" would be the strangest we would see.

If you disagree with this philosophy, which I think is more or less the philosophy of the moderators etc. (although please correct me if I'm mistaken), then as Gerhard says, you are free to set up your own system somewhere else.