A common used unnecessary step is the login step. Most people are not interested in getting account. Therefore it could be better removing this step.
Excellence Magento Blog created a good solution that removes the login step.
Remove the login step in Magento
Go to http://www.excellencemagentoblog.com/magento-onestep-checkout-remove-login-step and follow the instructions or like them on Facebook in order to get the complete file.