The best viral advertising clips have a very simple, but solid underlying idea, are well produced (which doesn't refer to cost, it's more about having an eye for the detail) and most importantly (really!) communicate the USP of the product in a clear, concise and unambiguous way!
My all time favorite for a campaign that fits this profile is of course Blendtec's Will it Blend. But here is a more recent example from BMW. I don't care much about motorbikes, but after watching this clip, I totally get what this product is about. Do you?