Index: wp-includes/formatting.php
===================================================================
--- wp-includes/formatting.php	(revision 24813)
+++ wp-includes/formatting.php	(working copy)
@@ -1169,10 +1169,16 @@
  * @return string Balanced text
  */
 function balanceTags( $text, $force = false ) {
-	if ( $force || get_option('use_balanceTags') == 1 )
-		return force_balance_tags( $text );
-	else
-		return $text;
+	if ( $force || get_option('use_balanceTags') == 1 ) {
+		$quicktags = apply_filters( 'balancetags_quicktags', array( 'more.*?', 'nextpage' ) ); 
+		// Outer capture lets PREG_SPLIT_DELIM_CAPTURE return the quicktags
+		// Inner noncapturing group matches any of the defined quicktags
+		$quicktag_regex = '/(<!--(?:' . implode( '|', $quicktags ) . ')-->)/';
+		$parts = preg_split( $quicktag_regex, $text, -1, PREG_SPLIT_DELIM_CAPTURE );
+		return implode( '', array_map( 'force_balance_tags', $parts ) );
+	}
+
+	return $text;
 }
 
 /**