After being a member of 1 years in SCN. I have seen many discussions (or documents, blogs) got rejected by a moderator. Sometimes users take it easy and do not post his query again, sometimes users ignore moderation and post his discussion again in the same way as he did, sometimes he check the notification and understand the issue and post it correctly or sometime users post a question in SCN support (or even other forum) as a question "Why my discussion got rejected ?"
Mostly it happens with new user because they can't able to understand that what he should actually do, They just find the website and post the question without checking the forum/space or without searching.
There are already lots of document, blogs which explain the things about a guidance of posting question.
Firstly the blog written by Mr. Jürgen, Dear Newbie - Welcome in SCN MM space. He has specified the blog as for MM space but I think it is applicable for all spaces. Here he has given FAQs section which is giving a very good guidance how to post a discussion and reply in a discussion.
Mr. Rob has written a blog Asking Good Questions in the SCN Discussion Spaces will help you get Good Answers about how to post a question. Here he has given a very good explanation about a question. 
In one sentence, you can get a summary of all these reasons for a post got rejected in this document The SCN Rules of Engagement | SCN
Also there are many similar documents / blogs which can give you a guidance to be much familiar to SCN.
Most of all contents got rejected for some few reasons :
Basic frequently asked and answered question.
Posted in a wrong forum.
Asking for step by step procedure, doc, video, presentation, link
Not an appropriate subject for a discussion.
Basic frequently asked and answered question : As SCN is not a new website, there are more than 3600000+ discussions in this forum. you just need to find your solution for your query. It is also discussed in The SCN Rules of Engagement, option no. 3.
You have these search options :
Google Custom Search- SAP Community Network Search
And the most common and helpful Google.
Also you can find a guidance from this document How to use SCN search
Posted in a wrong forum : This is a very common thing which has done by maximum users. SCN will accept all types of discussion which is related to SAP or related SCN improvement, Idea, query etc etc. You just need to find the specific forum / space. SCN has specified his forum as per SAP modules, here is the SCN Site Index. Where you will find all forums which are associated with SCN.
Suppose you have some problem is your computer or laptop, then you can't call a carpenter to fix the problem. You need to call a computer engineer. The same way, when you will face a problem for MM module, you can't post your query in FI forum. You need to post it in MM forum. Read the document Find Topic Spaces on SCN (Forums) for a brief explanation about how to find the appropriate forum which can be fit with your discussion / Document / blog.
Asking for step by step procedure, doc, video, presentation, link : SCN is a website where you can find all types of content like as document, blog, wiki and help portal. So why should we ask other people for a document as we can find it easily by little searching. Step by step procedure is well discussed in a document, blog, wiki and help portal. Some peoples are asking like that "Can anyone tell me the process of xxxxxxx", "Can you share me some documents for the process xxxxxxxx", "Can anyone tell me what is xxxxxxxxx". It is suggested to stop posting these types of discussion and start searching about the process and understand about that. Other users are ready to solve your problem, they are not here to find a link or document for you.
Not an appropriate subject for a discussion : I have seen lots of discussion got posted with subject of "Please help me", I have a problem", "Dear Guru help me", "I am facing an issue" etc etc etc. Do you really think it is correct ? Subject is the most important when you are posting a query, also when you will try to search by using any search term, the subject will be the first priority to search with regards to the keyword. Do you think anyone can search a discussion with the keyword "Please help me", I have a problem", "Dear Guru help me", "I am facing an issue" ? Message subject is a summary of your whole message. It should be always like a headline of a news which will introduce your whole discussion.
There are some more reasons why your discussion got rejected:
If you post your personal information like as phone number, email id in your discussion. You can see the details in The SCN Rules of Engagement | SCN, option 6.
If you will use some different language or SMS speak language in your discussion, You can see the details in The SCN Rules of Engagement | SCN, option 4.
You can find some more reasons in this Blog post. Mr. Matt has written it very clearly and well explained.
Most of the people (mostly for new users) is not familiar with the tab Action and also little bit about Communication. That because they are ignoring moderation and post his query again in same forum and same query.
An user post his query and just follow up the discussion, if the discussion got rejected then he can't able to see the discussion neither his profile - content nor space - content. Then just one question comes into his mind "Where is my discussion ? 
"
When a content got rejected by an SCN moderator, then SCN will send a notification to you about your content rejection.
What is the first step when a content got rejected ?
In any page, you will find your three tabs when you will log on to SCN. here it is :
The activity contains this information which you are following. The Communication contains this information which is communicated with you. The action tab contains this information of some alerts or some notifications.
As I have told that SCN will send a notification when your content got rejected by a moderator, then you need to see the action tab.
If a new notification will appear in Action tab (even any of those tabs), then you can see the highlighted green colour number on your tab. You need to click on the green number to see the notification. After clicking the action tab, You will find a notification tab (with highlighted green colour number). Click that notification tab to see the message.
Now you can see a formal message as
Your content "xxxxxxx" has been rejected by an SCN moderator and is no longer visible in the community. Please see the Rules of Engagement for more information on why content is rejected.
Then below of this message, you will find a Reason option. Here you will find your reason why your content got rejected.
It will be looks like below:
Now the reason is in front of you. Read the reason carefully and follow the the same.
What If I can't see the reason option in my notification tab :
That means the moderator hasn't given any specific reason for the rejection. Generally all moderators give the reason for a content rejection, sometimes moderator think you need a brief explanation/guidance about the content rejection. Then he will send you a direct message.
You can see the direct message by clicking the Communication tab.
What I do if I am not able to see any reason in notification or any direct message :
Generally all moderators give the reason of the content rejection or send a direct message to you. In case, if you didn't receive any of those then first you need to read and compare your content with The SCN Rules of Engagement | SCN and check if you are violating any of those rules which is pointed.
If you strictly think that your content is not violating any of those rules, then post a query in SCN Support forum. People (along with moderator) will get in touch with you. But remember do not post only the message like that "I have posted a query in space xxxxxx, but my query got rejected. I want to know the reason". Give a brief explanation that you have compared the all points of The SCN Rules of Engagement and it is not violating any of those rules.
What should I do if I an not able to see any notification or not any highlighted green number in action tab although I am not able to see my post in the space :
Sometimes it happens like that you haven't received any notification and also you can't able to see your discussion in the space overview or content. This will happen when another user will press Alert moderator for your post. If one member think that your content is violating any one of the rules of SCN, then he has the ride to press Alert moderator for your post. if your post got one Alert moderator, then your post got removed only from space's content but it will be visible in overview tab. If your content get one more Alert moderator, then your post will be removed from overview and content both tab. In that situation, you will not receive any notification in action tab as because your post has just got multiple alert moderators but not yet rejected by a moderator. If the moderator will accept your post, then your post will be visible. If moderator will reject your post, then you will get a notification under Action tab. So when you will face that situation where you can't see any notification under your action tab and also you can't able to see your discussion in your content tab or in space's content / overview tab, then you do not need to worry about that.
Finally, I'll suggest to all new and old member that please do not ignore moderation and do not post your query again once it got rejected by a moderator. Follow the simple steps and be a better member in SCN. Follow and respect the SCN rules and encourage people to do the same. Remember if you will behave better in SCN, then SCN (along with all peoples) will serve you better.
If you have any query or need more explanation about any related content, then you can contact me, you will find my contact details in my profile.