Most of those questions that you list are not good representatives of what we should strive towards, and would likely be closed as too broad if they were posted now, that the community has more-or-less spoken what's on topic on our site. One thing that needs to be considered when giving such examples is, that most were asked during first few weeks of our beta, some even during private beta days. Our scope wasn't yet refined, we didn't have our own moderators, community managers didn't want to interfere too much with how we evolve, and there were many such canonical questions posted for various reasons. Most of these were actually much discussed, but let's see what issues they could have raised, if they were asked now (going by their titles alone):
- Which planet or moon would be most feasible to terraform? Too broad, and if feasibility mentioned in the title isn't well defined in the question's body, it's likely also primarily opinion based. In a nutshell, the title reads as an invitation to a discussion. Good for visibility, bad for getting well researched and to the point answers on a specific question.
- Would terraforming Mars be possible? Does the question suggest why it wouldn't be? Or how one might want to do that? If not, then it's too broad, and again, also primarily opinion based since answerers are left to their own devices when selecting ways of achieving it.
- What would the size and rotation of a station need to be to produce 1g gravity from head to toe? Sound fine, it's specific enough that it can be answered without further ado, and still leaves an opportunity to discuss implications and actual requirements for human spaceflight.
- How would it be possible to kick start Mars's magnetic field? Doesn't sound too encouraging, I'd expect either prior research and substantial explanation in the question's body, at least listing which means are at one's disposal and which aren't. I'd be tempted to comment with "Place it in a huge microwave oven and set to thaw". But I'll resist temptation.
- What is the smallest number of humans required for colonization? I remember this one stirring quite a discussion, and I eventually edited it to reword slightly sensitive language. But it's actually quite an interesting question, especially after successfully narrowing it down to be specific and clearly identifying which conditions an answer should meet.
- What is the largest naturally occurring body that could be hollowed and safely lived in? Similar to the previous one, and after clarifications, it actually got a satifying answer. It's likely still a bit too broad, but it's hard to argue a good answer. It also must have been closed and reopened a few times. It's fringe science, but not necessarily infeasible even current technological reach. Of course, if we'd be at the level of self-replicating machinery,...
- How would manned interstellar travel become feasible? Entirely opinion based, and way, way too broad. One could write a book on the topic and not cover half of it, while still not actually answering anything. There simply isn't anything to base feasibility on, or substantiate claims in any way. 100 Year Starship? Developing FTL? Something else we didn't yet discover or have no experience with?
So these are some initial thoughts on these questions, mostly going by their titles alone, unless I actually remember them, in which case I had a bit more to add. I was intentionally shallow here, not to go too deep into each individual question and write another incredibly tedious answer. But hopefully my commentary to them will help you appreciate better what we're looking for. Specific questions that warrant specific answers. It is OK to intentionally leave one end slightly ajar to allow additional insight at answerers' own discretion, but that should be optional and not a basic requirement for an acceptable answer.