I have recently started using Markdown in a project using a Mantis
tracker, and due to the syntax for quotes text in lines starting with
">" I would strongly recommend using this instead of reStructured text
or Creole.

One thing I missed in the Mantis implementation was a preview mode to
see how my entry will be rendered before submitting it. This is even
more important here, because Roundup does not (yet) allow editing messages.