On a site with as much history as Stack Overflow, defining the exact boundaries of what is on- and off-topic can be a real challenge. Rather than trusting my gut on this, I sat down and analyzed a random sampling of recently-closed questions, wrote up a handful of reasons that covered the bulk of them, and then threw them at the moderators for feedback. After some discussion and several revisions, this is what I'm suggesting for the initial rollout:
Custom off-topic close reasons
Questions about general computing hardware and software are off-topic for Stack Overflow unless they directly involve tools used primarily for programming. You may be able to get help on Super User.
this was the most commonly asked off-topic topic, even excluding questions that were eventually migrated
Questions concerning problems with code you've written must describe the specific problem and include valid code to reproduce it. See http://sscce.org/ for guidance.
Not every question involves a specific chunk of code, but if it does that code must be included, as short as possible, and understood well enough by the asker to allow for a descriptive title; "here's my code, find the bug" questions should be closed ASAP - this covers the bulk of reasonable Too Localized closures.
Questions must demonstrate a minimal understanding of the problem being solved. Tell us what you've tried to do, why it didn't work, and how it should work. See also: Stack Overflow question checklist
this complements reason #2 for questions that don't involve already-written code. If you're asking others to solve a problem for you, you must be able to understand and communicate it well enough to allow useful answers to be written and found by others.*
Questions on professional server, networking, or related infrastructure administration are off-topic for Stack Overflow unless they directly involve programming or programming tools. You may be able to get help on Server Fault.
There's no longer a migration path from SO to SF, but that doesn't stop folks from asking system administration questions on SO. Disturbingly, a lot of questions on programming tools get closed for this reason because certain folks think development servers are at all like the real deal - this might help to discourage that.
Questions asking for tool or library recommendations are off-topic for Stack Overflow as they tend to attract opinionated answers and spam. Describe the problem and what has been done so far to solve it.
This one is long overdue
I should also note that once the new system is rolled out, we'll be able to query and analyze the reasons provided via the "other" off-topic option in a fairly simple fashion - I fully expect to be tweaking and changing these reasons fairly quickly based on observed usage. But that doesn't mean we shouldn't try to hit as many common mistakes as possible right out of the gate, so...
Thoughts? Improvements? Concerns?