Design questions are ones that deal with architecture, package layout, object hierarchies, etc., but do not necessarily include code.
We already deal with this sort of question on a daily basis at programmers. While a certain amount of overlap is inevitable (and desirable, see this Stack Exchange blog post), this could be a big subject.
Should on-topic be limited to actual code, or can design be reviewed here as well?
In my opinion, Code Review should strive to create cleaner, more readable, "better" code and architecture / design / design patterns are a part of that.
switch (object.type): case "Foo": doX(); break case "BAR" doY(); break; .....
is something that could be solved using "design" (inheritance).
Also I would like to be able to post question like "I've build this object hierarchy and now it bites me because X, Y and Z. And by god I have no idea how to refactor that into something that changes me problems".
The same thing that I would to in a real life code review when going over some new module.
The Code Review Stack Exchange is for sharing code from projects you're working on to get peer feedback and review. There may be rare exceptions, but the bulk of questions will likely contain actual code you are working on.
The Software Engineering Stack Exchange is for "white board" programming questions. Higher-level design and architecture questions should be posted to Software Engineering SE.
Incidently, questions about the discipline and practice of code reviews should be posted to Software Engineering SE. If you see any questions about conducting code reviews (best practices, techniques, etc), those questions should be closed and the author kindly directed to the Software Engineering SE site.
I personally think that code should be involved. Anything that is purely a design question should go to Software Engineering.
