I agree that it is quite easy to distinguish between open and locked thread, and each friendstertalk forum theme has its own style and image for locked thread.
For example on the default one,
normal threads will have this icon [img]http://theftalk.com/img/FriendsterTalk/icon_normal.gif[/img] or [img]http://theftalk.com/img/FriendsterTalk/icon_new.gif[/img] with blue title link,
locked threads are using this [img]http://theftalk.com/img/FriendsterTalk/icon_closed.gif[/img] with black title link.
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BTW how many members told you about it?