There are two types of visionary in the world: the specific and the general. Bill Gates is a general visionary. Years ago he predicted that the next big thing in computing would be tablets, and he was right. But being right about tablets didn't allow Microsoft to create a form of tablet computing that anyone wanted to use.
Steve Jobs was a specific visionary. He famously said about tablet computing, "If it uses a stylus, you're doing it wrong." His vision was of specific things: not the tablet, but how the tablet would work and how you would use it.
Each have their place. "Some day polio will be nothing more than a memory" is a general vision, but in the right hands it's a vision that can and will change the world for the better. Specific visions, though, are the ones that help you build great products. They focus your attention; they tell you what not to do.
General visions give you the Gates Foundation. Specific visions give you the iPad.