I know that there has been previous discussion about the inappropriateness of certain questions for mathoverflow.net, such as questions pertaining to elementary notions of compactness or differentiability. So, in a sense, there is an established lower bound for the difficulty of questions for mathoverflow.net, but is there an established upper bound for the difficulty of questions for math.se? Certainly, this is de facto the case, but is there a level of question which would be inappropriate for math.se based on the high level of difficulty or sophistication of the question? If there is not, should there be? It seems that it would be detrimental for certain high level problems to be posted on the site, and certainly not beneficial to the majority of users of math.se.
All mathematics problems are welcome here. There is no upper or lower bound for what level question may be asked, and I do not see why more advanced mathematics would be "detrimental" to the majority of users.
In some cases, after receiving no answers, some users have then posted their question on math overflow. In some rare cases where a questions was posted on both sites, the answer has actually appeared on math stack exchange rather than overflow.