I find that there are at least six types of questions that the community on Christianity.StackExchange has decided are on-topic and within site guidelines. These question types, if the few rules that accompany them are followed, almost always remain open and receive high quality answers.
Christianity.StackExchange Proven Question Types:
Questions usually need to ask for a perspective. Who do they want an answer from? Example. You see in this question and my answer to it, there is a very strict Roman Catholic frame. The question is about a Catholic dogma and my answer uses the official Catechism of the Catholic Church to answer it.
Some questions are about a perspective (or opinion or denomination) in general. Example. In this question, the author knows that 7th Day Adventism is different, but wants to know how. My answer clearly and factually addresses that; I tell him what SDA's believe that is different from "mainstream" Christianity.
Some questions are about factual history. Example. This question wants a straight forward, historical account of when and how a certain theology developed.
Some questions are opinion history. Example. This question is technically asking for something that we cannot factually answer, but we can factually support it. My answer uses quotes from Augustine's own writings to support my conclusion. This one is similar, but is different in that you would need to use factual points about historical circumstances to support any conclusion. Naturally, there may be disagreements that ensue in the comments. The opposing party is always welcome and even encouraged to provide his opposing view in his own answer.
Some questions ask for the "biblical Basis." Example. It is vital to note that a good answer to this kind of question mandates that you provide the verses and arguments used by proponents of the view in question. Whether the verses are being interpreted correctly or not by proponents of that view is irrelevant to this site. Another example here.
Similar to "biblical basis" is asking why a certain Bible story happened the way it did. Example. This is like the opinion history in point 4 above, but is relative to the Bible only. It is difficult to support these questions with outside sources, but usually are answered well by spelling out exactly what happened and how the culture and times might have affected it. Here is another example. And another. And another close one. It is a popular format and is usually asked by those curious for more details about last night's Bible study.
After all these are the others. These are on a case by case basis and you need to be careful how you word things or it will be closed. It seems that eschatology is almost always off-topic. Example. Another Example. Christian culture is almost always off-topic as well (though I quarrel with that). Example. Another Example. Other off-topics are "What should I do?" or "What is the Christian thing to do?" Those are pastorial advice questions and are easy to spot. If it is not directly about Christianity then it is off-topic. Example. Also, questions without a frame asking for what is true ("Truth questions") are off-topic. Example. An ironically broad off-topic question type is one that is too broad. The scope is simply too broad to answer succinctly and effectively on this site. If you asked a question that was closed as too broad, but don't necessarily know why, it is simply because the topic is way more complicated then you originally thought.
If you can follow the format of the question types 1 through 6 then the community finds it acceptable (usually) and it will remain open. For answering these questions at least follow this one rule always: If you can quote the Bible or something else to support your answer then do it. Question types that fall under "other" (type 7) are risky. I would suggest that you do not try to ask or answer any questions in that type until you are more familiar with the site and how the community reacts to certain subjects.
Also, remember that everything about Christianity is somebody's opinion. Here on this site we try our best to stick to describing those various opinions. We try not to take sides and appear like the site supports one opinion over another. That is the single unifying point for all of the above, on-topic question types. Always ask and answer as objectively as possible.