There's a depressing number of postings in clients-from-you-know-where dot com involving people who have to be told "Uh, no, that's not Photoshop. That's magic. It's in a Harry Potter movie."
Does the js execute before the whole (original) page loads up? That's got to be the single biggest issue with js. Well, apart from users who have it disabled. (Can you turn it off on a phone? You can on the iPad-- but that doesn't need "mobile" site versions.)