From the blog post Introducing Programmers.stackexchange.com
In a nutshell, Stack Overflow is for when you’re front of your compiler or editor working through code issues. Programmers is for when you’re in front of a whiteboard working through higher level conceptual programming issues.
The tag description for the design tag is:
Design in programming is the act of making particular choices about how best to structure a program, to achieve goals of reliability, maintainability, correctness or ease of use.
As far as I can see, everything in this description falls into the category of 'higher level conceptual programming issues', and nothing falls into the category of 'code issues'.
While I don't rule out the possibility of writing a question which is about design and which is appropriate for this site, I think that almost all questions tagged with [design] are better suited to Programmers.SE. Is it time to remove that tag?
Possible feature request: notify a user who posts a question with the [design] tag that such a question is probably better suited to Programmers.SE. Math.SE already notifies users when a question is likely to be too subjective for the site (based on some strange computer interpretation of its title), so this could be something similar (and more likely to be correct).