This is almost essential.
ExpressionEngine's major advantage over WordPress is a third-party plugin, developed by Brandon Kelly, called Matrix.
Using jQuery, Matrix allows custom field placement inside a data table, to which rows can be added, subtraced and reordered with the click of a button. This means that developers can implement features with extraordinary ease (for instance, client-updatable tracklists that feature lyrics, aand 'buy' and 'preview' links for a record store).
The uses for this kind of plugin are endless.