In the beginning, Stackoverflow contained all kinds the questions, including the famous "comics for programmer" type of questions.
Then programmers.stackexchange.com was created, and all that stuff was migrated here, so only "professional" questions remained at stackoverflow.
Then came the disciplining, and now programmers.stackexchange.com has become another place that only allows professional questions.
There is a distinction between "objective" (so) and "subjective" (p.se) questions, but I think that this is a bit arbitrary. In fact, questions migrated from so are often closed in p.se as well; and there are non-closed questions on both sites that could be asked on the other site as well.
Examples (all from the first page as of this writing):
How do I implement the MPL? What's this "Exhibit A" thingy?
Are MSDN licenses transferrable? How about if they have expired?
A possible solution to that dilemma would be to restrict SO to pure technical/coding questions. Currently, most of the questions on SO fall into that category anyway. That would require a change in SO's FAQ, though, since it reads...
... if your question generally covers …
* a specific programming problem * a software algorithm * software tools commonly used by programmers * matters that are unique to the programming profession
… then you’re in the right place to ask your question!
The last item on the list includes a lot of things that are allowed on P.SE too.