Index: wp-includes/nav-menu.php
===================================================================
--- wp-includes/nav-menu.php	(revision 18733)
+++ wp-includes/nav-menu.php	(working copy)
@@ -618,8 +618,12 @@
 			$menu_item->target = empty( $menu_item->target ) ? get_post_meta( $menu_item->ID, '_menu_item_target', true ) : $menu_item->target;
 
 			$menu_item->attr_title = empty( $menu_item->attr_title ) ? apply_filters( 'nav_menu_attr_title', $menu_item->post_excerpt ) : $menu_item->attr_title;
-			$menu_item->description = empty( $menu_item->description ) ? apply_filters( 'nav_menu_description', $menu_item->post_content ) : $menu_item->description;
 
+			if ( empty( $menu_item->description ) ) {
+				$description = wp_trim_words( $menu_item->post_content, 200 );
+				$menu_item->description = apply_filters( 'nav_menu_description', $description, $menu_item->post_content );
+			}
+
 			$menu_item->classes = empty( $menu_item->classes ) ? (array) get_post_meta( $menu_item->ID, '_menu_item_classes', true ) : $menu_item->classes;
 			$menu_item->xfn = empty( $menu_item->xfn ) ? get_post_meta( $menu_item->ID, '_menu_item_xfn', true ) : $menu_item->xfn;
 		} else {