No, you can't have a definition for subjective. Because it's subjective. That's what troubled SO for so long, and eventually lead to the banishment of the subjective tag - no one could agree on what it meant.
Time isn't going to fix this. Discussion isn't going to fix this. Not in any reasonable time-frame at least - or it would already have been settled on SO.
If "subjectiveness" is the criteria by which questions are judged, then everyone's just going to argue that their favorites are on the low end of the scale...
...But the current arguments aren't really about "question subjectiveness" anyway. Some of the commonly-cited "good" questions are extremely subjective. The problem is that the obvious rules for separating on- and off-topic questions (strictly programming related, no GTKY/write whatever you want) exclude some rather popular questions - so instead, you get these arguments in favor of more (ahem) subjective criteria:
- "potential for long-term benefit to humanity"
...and those are at least as difficult to nail down as "subjective".
So to answer your second question: forget it. You're not gonna capture this in the FAQ in any useful fashion. I say we just admit this right at the start, and codify it thusly:
What kind of questions can I ask here?
Questions about things programmers care about. Good ones.
How do you know if your question is good enough for Programmers.SE? You don't. Question "goodness" is hard to define, and so we're not gonna try. But we know it when we see it, and if we don't see it, you're outa here!