Index: src/wp-includes/css/media-views.css
===================================================================
--- src/wp-includes/css/media-views.css	(revision 29221)
+++ src/wp-includes/css/media-views.css	(working copy)
@@ -931,8 +931,14 @@
 
 .media-grid-view .attachment .check {
 	display: block;
+	opacity: .3;
 }
 
+.media-grid-view .attachment:hover .check,
+.media-grid-view .attachment.selected .check {
+	opacity: 1;
+}
+
 .media-grid-view .attachment .check div {
 	background-position: 21px 0;
 }