Why does everybody keep saying to Jon Snow that he knows nothing in Game of Thrones? What do you think he knows?
(IMO) What doesn't Jon Snow know? Too damn much!
(I've decided to spoil mark this answer. Even though it is my opinion, it may reveal information not yet known by other viewers, understanding that GoT is an unusual case.)
Old Nan recounted a story to Bran about an old king of the North (a progenitor of the Starks) who fell in love with a Queen of the White Walkers. He became King of the North. His abilities (to understand the language of, as well as understand the ways of) the WW's has been passed down through his descendents.
Jon is not the bastard of Ned Stark and Ashara Dayne, but rather Lyanna Stark and Prince Rhaegar Targaryen, making him both heir to the Stark line's abilities as well as to the Targaryen's.
When Jon meets a dragon, those abilities will be re-kindled in him. I believe he will have the White Walkers following him while riding atop a Dragon.
Who his father really is..
Who his mother really is..
3. That he will be able to understand, and lead, the White Walkers..
4. That he will be able to understand, and lead, Dragons..
(#2 is (sort of) common knowledge, at this point, although who he believes his mother is, may not actually be so...)