The dialog is very easy to use for simple rules. I think it's basic form has to retain that simplicity, or more confusion would result. Maybe the default AND brackets should always be there—maybe in blue as a hint they can be changed. Or some other visual cue that AND/OR brackets can be set.
As Vincent points out, it's also helpful to realize the expression can be constructed using the interface, or entered directly in the "Import/Export" box. If having difficulty with one, you can use the other—and then see the result in the first. I use that far more often as an expression editor than I do as an import/export tool. It should be turned into one (i.e., by including the usual insert field/function tools) and called "Expression Editor - Import/Export." That would make it more apparent it can be used for either purpose.