>there is no such thing as a stupid question or an irrelevant one

I think that's where they disagree with you. Stack Overflow aims to keep discussions as minimal as possible and is not the place to weigh the pros and cons of programming language X against that of programming language Y. If you want more information, they cover their take on the issue in great depth on their podcasts (which are great to listen to, regardless).

I like what they've done. By closing all bad questions and a few good questions the site contains only good questions. The obvious downside is that the occasional interesting question is removed, but I think the trade-offs are worth it. And judging by the popularity of the site, I think most everyone else does too, even if they're not consciously aware of it.

SO would suffer, because the kind of question you're talking about is intrinsically debatable. Those questions aren't bad or wrong, but they are inappropriate for the medium of SO, which is about technical questions with concrete answers. SO is not the be-all, end-all of knowledge, but it is pretty close to perfect for technical questions with right answers, and I think it would suffer greatly if it developed an HN or reddit-like clubhouse atmosphere.

A "not relevant" question is usually not a "bad" question, but just a question that can't have a good, definitive answer. SO are trying to be a site hosting such answers, not discussions.

So opinion-based questions are explicitly unwelcome on SO. They can collect lots of worthy information in comments, so such questions are usually closed but not deleted.

SO even tried to channel the discussions by developing discourse.org so that interested parties can get a them a discussion forum and continue there :)