I'm not convinced Joseph understood the implications of his own visions and revelations when they happened. We have a hard enough time contemplating them even almost 200 years later; I think he learned "line-upon-line" from each subsequent experience. I think he understood his First Vision better through the years as it was reinforced by other experiences, so . . .
I'm not sure if the Lord told him not to reveal all of the First Vision details or if he simply didn't understand it all, but I think it's a combination of the two. I think he was overwhelmed by it; after all, Moroni later had to repeat his visit three more times after the initial vision in order to be sure Joseph understood it all. Of the two, I'm fairly certain that the First Vision would have been more overwhelming and hard to fathom than Moroni's message.
When you boil everything else down to the most fundamental principle of the Restored Gospel, I think it can be stated as: "I am a child of God." The literal reality of that statement is mind-boggling even for us, much less for others still bound by the teachings of centuries - as Joseph undoubtedly still was even after the First Vision. It was and is the heresy that nearly got Jesus stoned; Joseph had enough trouble with his claim of translating the Book of Mormon; I think understanding the First Vision fully and broadcasting it publicly easily could have killed the Restoration before it got started.
That's speculation, of course, but it makes sense to me.