On Question/Answer social applications

I like question answer social applications like StackOverflow.com because most of the times they are a better alternative to a forum. One of the creators Joel Spolsky explains the success at Learning from StackOverflow.com video.
The ideas that make StackOverflow.com so great are summarized at:

• Voting: There needs to be a way to rank answers to a question up or down so you can see the best answer the first.
• Tags: Hierarchical categorizing fails where tagging works wonderfully.
• Editing: You need to be able to edit both questions and answers so they do not become obsolete.
• Badges: Users should gain status by completing actions on the site that gain knowledge on the site features and make people use them.
• Karma: Users should gain status by asking good questions and replying with good answers.
• Pre-search: Asking a question needs a search first. It really prevents duplicates.
• Google is UI: Google should be able to index the whole site for users to access questions.
• Performance: A performant site is used.
• Critical Mass: Users will only use sites that trust on. And stackoverflow.com was developed by two very influential IT bloggers.

StackOverflow.com also has major problems like not accepting any kind of question that make those questions rot since they cannot be updated anymore. Either they should be consistent and remove them or just allow discussions under a different site or with other tag. Stackoverflow should also suggest the posters of those closed questions to post to Quora instead.