gistfile1.php

<?php

// $class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item, $args ) );

add_filter( 'nav_menu_css_class', 'wsuspine_fix_nav_item_classes', 10, 3 );

function wsuspine_fix_nav_item_classes( $classes, $item, $args ) {

	if ( in_array( 'current-menu-item', $classes ) )
		return array( 'current' );

	return array(); //should return an empty array of classes.	
	// return array( 'my-custom-class' ); // returns a custom array of classes
}

So, this is close. It's eliminating all the extra classes but failing to return .current on the current menu item.

For my own reference: http://codex.wordpress.org/Plugin_API/Filter_Reference/nav_menu_css_class

Beautiful! That's it. Can't I give you a brownie point like on stack overflow? Shame.