Customers are looking for something equivalent to the Hudson/Jenkins function:
https://wiki.jenkins-ci.org/display/JENKINS/Parameterized+Build

As a plan creators they want to be able to say:

"These three variables are required and these two variables are optional."
When the build is run, a form comes up, showing the variables to the user:

Example:
Release name: (Pull down of multiple release names the job creator has defined)
Please enter the name of the release you want to build from the pull down above.

Your SVN user id: (Pull down from valid user ids or free form text)
Please enter the user id you use for SVN.

Your SVN password: (A hidden input field)
Please enter your SVN password above.

Additional users to notify: (Multi-pick field based on users of system.)
Please add any additional users you would like to notify.

Project to charge build to: (Pick list defined by plan creator showing cost centers.)
Please enter the cost center for this build.