Your concern seems to be more a matter of the thoroughness and specificity of questions, rather than "subjectivity & scope."
The FAQ outlines the scope of topics discussed (updated as needed), while the Good Subjective guidelines deals with issue of qualia ("what sort" or "what kind").
But there is a whole class of "generalized questions" appearing on this site which are not well-suited to this type of Q&A site. They are those discussion-class questions where the users doesn't have a specific problem but only wishes to open a dialog to discuss generalizations and broad issues.
- What is the best … ? What is your favorite … ?
- What are the (dis)advantages of … ?
- Is [X] good? Is [X] bad?
There's nothing inherently wrong with those questions — they have been asked hundreds of times on every other forum on the subject — but in the context of a Q&A system like Stack Exchange, they should be discouraged.
If users don't provide enough information and ask specific questions about a problem they actually have, the answers will be somewhat generalized — not very authoritative. Everyone kicks in their thoughts on the subject until the topic has completely exhausted all possible interest from the participants. What is left behind (for those who come after) is a lot of good information, only it is buried in noise and the random opinions of the conversation.
That's where the problem of subjectivity lies. They're not "real questions". They're "hey, we should talk about …" discussions about broad topics, disguised as questions posed for entertainment.
Stack Exchange is specifically designed NOT to handle these overly discussion-y questions very well, but it also takes community self-moderation to guide users and keep them on track. Keeping each question focused on a specific problem at hand will keep the content highly relevant and it will continue to to solves people's problems. That's what makes the collection of knowledge you build here worthwhile. It has a certain longevity not found in other discussion forums.