First off, thanks! Criticism is always important to be heard and hashed out.
That said, I think you may be a bit off-base with some of your criticisms. Here are a few things (not necessarily all) that come to my mind in response to your post:
We naturally put limits on the sorts of questions that can be asked here; you seem to accept this when you talk about banning questions on football or celebrity gossip. What, then, should be the limits we put on questions posted—and why?
The point of a StackExchange site, as we understand it, is primarily to provide objective (in some sense) answers to questions; thus, regardless of why people might come to the site, we have a specific reason for existing. We therefore usually accept only specific types of questions that in our experience we can answer objectively.
The criteria we have for what's acceptable as a question aren't subjective, nor are they arbitrary; they've been arrived at after a few years of experimentation. They've allowed a good number of people to ask and answer some interesting questions, and they seem to be working for most people. (Though it does also seem that most people do take a few tries before they really get it. I'm not sure this is avoidable.)
Let's start with something you seem to have difficulty with: the idea that "posts cannot discuss 'truth' or debate who is a real Christian, but must accept all beliefs". This follows on from the fact that as a StackExchange site we are here primarily for objective answers, not for debate or discussion. If I answer a question from a Catholic view, another person from a Southern Baptist view, another from an Orthodox Christian view, and a fourth from a sedevacantist Catholic view, which one should be accepted as the objectively true answer? How would we arrive at an agreement on the question "What is 'Christianity' and who is a 'real Christian'?" Chat is designed for that sort of discussion and debate, and we do have those discussions in chat. But on the main site, we can't provide an objective answer without coming to an agreement on those questions, and we have found by experience that it appears impossible for an arbitrary group of sincere believers to agree on that question in any way except the way we have, namely by stating that any group who calls themselves Christian is considered Christian for the purposes of the site. (This by the way addresses your question whether one should accept someone who claims that Muslims or Buddhists are Christians. Since neither group claims to be Christian, they are not considered Christian for our purposes.)
You give an apparent counterexample of someone being unable to run a "Java" program, posting on StackOverflow, and being told that what they had posted was not Java—according to the international standards committee responsible for Java. However, there is no such committee for Christianity, and there are an incredibly broad variety of views within it. We can't discuss what is "valid Christianity" in the same sense one can discuss what is "valid Java" simply because there are tens of thousands of groups, all with different (sometimes wildly different) beliefs, all of whom believe that they have the unique "valid Christianity".
What we've found is that the only way forward, the only way not to have questions tail off into endless lists of alternative viewpoints (none objective in any sense) is to ask primarily not what is true, but what various groups believe to be true, and why they believe them to be true. That's what gets enforced here, so that we can provide the kinds of answers the site was designed for.
As far as questions being closed: "closed" is a relative term, and always subject to reversal. We put debatable questions "on hold" before we close them, we don't close questions that seem to fit within our usual criteria, and we have this very Meta site for discussion of specific closed (or on-hold) questions that someone believes should be reopened. After review and a voting process by relatively high-reputation users, any question can be reopened if it seems to suit the site. We invite the user to come back, re-edit, and request reopening.
Edit— One other thing: You're asking about what it would take for this site to be made successful. Another poster has commented on the number of hits to the site—the number of people who visit. As far as the ability of people to participate, given the rules you feel are too restrictive: based on a quick Data.SE query, we've averaged about 3.5 unclosed questions per day, and 12 answers every day since the site was opened 4⅓ years ago. While miniscule compared with (say) StackOverflow, these numbers don't seem to indicate that people have problems with posting questions or answers while keeping to the site's rules.
I'm not sure I've answered all your questions here, but those are the first couple of responses that come to mind :-)
