Index: wp-includes/formatting.php
===================================================================
--- wp-includes/formatting.php	(revision 20239)
+++ wp-includes/formatting.php	(working copy)
@@ -1062,7 +1062,12 @@
 function balanceTags( $text, $force = false ) {
 	if ( !$force && get_option('use_balanceTags') == 0 )
 		return $text;
-	return force_balance_tags( $text );
+	$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));
 }
 
 /**