best package for given website functionality

I aim to build a website with the following functionality:

• The site should allow users to post to it with other users having the ability to comment on their posts.
• each user should have a username and password
• each post should be listed under certain criteria (as in song genres etc.)
• the content of the site should be accessible by one or more given classifications.

I guess for the last point, the use of keywords or tags in posts would allow for this feature. But what about the case where the user does not provide any keywords or tags? The site should have the ability to classify content or extract keywords and tags automatically.

What is the most appropriate tool or software to incorporate this functionality?

I have been exposed to the Mojo marketplace packages which can be deployed free of charge. Can any of these packages serve this purpose?

Some of these packages can incorporate forum functionality while others are CMS systems like Wordpress and Joomla.