WikiLinks is a popular extension that provides support for easy linking to wiki pages. For Confluence, the base support has been extended to include the Confluence wiki linking syntax to allow linking to pages in other spaces and attachments.
Markdown allows HTML to be intermixed with markup to enhanced rendering capabilities. However, allowing embedded HTML can be a security exposure on your site if untrusted users have access. All Markdown macros have an allowHtml parameter. This parameter can only be used if CMSP has been installed and configured to allow the parameter to be turned on.