When editing rule parameters directly from the quality profile page (i.e. not going via the "Edit rule" button), there is a magnifying glass button which enables one to switch to a textarea (+ there seems to be some logic to do the switch automagically).
But, on the "Copy rule"/"Edit rule" interface, there is no way of inserting new lines in the text field, as it will trigger the page submit.
For some parameters, such as an XPath expression for instance, it really makes sense to always have a textarea.
Currently, when setting a parameter to "empty", the default values is forced back.
This is probably a bad idea, as Sonar is trying to be smarter than the user.
Instead, it would be better perhaps to display the default value in grey, in a similar way to the description.