We use the variable dialog box a LOT, and have the following suggestions to make it easier to use:
1- it should not be modal
2- it should remember its last size, at least within the session, the way the expression box does
3- it should allow the user to increase the size of either the variable expression field or the initial value expression field, relative to the other (if I'm working on a variable expression, I don't necessarily need the initial value expression field to take up a lot of space in the dialog)
4- the name field should be a combobox so that I can switch from variable to variable without having to close the dialog (it could work the way the element properties box does, as long as it isn't modal, so that I can click on a variable in the variable list and if the variable box is already open it switches to the clicked variable).