I agree that "open" is vague. There's nothing about it that makes me want to click on it. Now, if it read something more like "surprise me", I might be more likely to click on it. ;) But even so, then you're essentially trying to trick people into clicking on it in the hopes that they might decide to answer some orphan posts, which (to me) doesn't sound like a very solid approach. Also, anyone who might deliberately want to help out and answer some unanswered posts has to somehow guess that "open" means unanswered, so this actually makes it harder for those people.
What can we do to get people to notice there are a growing number of unanswered messages and get them to help out in answering them?
I agree with what BeeDeeDubbleU wrote. The menu is so small as to seem unimportant. Give some weight to the unanswered messages. I like BDDU's suggestion about giving them some focus at the top of each forum for a period of time. Or what about doing something like dedicating a small section of each forum to "feature" some of the unanswered threads? Another option might be to dedicate a small sidebar to showcase unanswered threads (though that would be a pretty significant change to the site layout, and would probably warrant some research and design work).
In any case, I don't think "open" really helps long term. Changing the link text may have created some short term buzz here, but I don't think you'll see long term gains from it.