What are the main issues with regards to mashups and copyright law?

Copyright Law - The exclusive rights of the copyright owner over acts such as reproduction/copying, communication, adaption and performance - unless licensed openly - by there very nature reduce the ability to negotiate copyright material without permission.

The permission that is required is that of a voluntary open content licence, Creative Commons licence, or if a statutory licence exists, or obtained straight from the copyright owner.

Most of the content of mashups are sourced from third parties who have not been given permission and therefore will inevitably encounter legal problems when the whole or part of the original material has been reproduced in some way.