It would be nice to be able to automatically create a site property in the expression editing window. Like when you're writing an if statement and you think it would be better to have a default value defined in a site property.
Now we have to close the expression window, go to the Data Tab, create the site property.
It could be the last option in the list of suggestions, like "Create a Site Property with this name and type". And determine its type automatically too, by the data type of the left side of the comparison, if any.