This is a very interesting question.
I actually wrote the Mistborn trilogy straight through before releasing the first, so I have some experience doing it both that way, and releasing one book before the next is written. With Stormlight, I’m much more careful with my foreshadowing. Maybe to the point of teasing. That’s a contrast to Mistborn, where I may have been too blunt with my foreshadowing. (Or just not put it in.)
The trilogy there was one book in my mind, so things that happened at the end of the first book that should have been better foreshadowed didn’t get the foreshadowing they deserve–because I was looking at them as elements I was introducing 1/3 the way through the story, and thinking of them as being on a proper curve of information.
The balance of what to provide and what to withhold has more to do with not bogging down this story with details for a future story than it does with trying to tease. In my mind, The Way of Kings is three things: Kaladin’s experiences as a bridgeman 2) Dalinar’s decision to do what he does at the end of the book 3) Shallan’s first apprenticeship. I wanted to keep the narrative focused on those things, and provide climaxes dealing with those three concepts. Other secrets and teases are more intended to begin setting up future stories.
However, the “Lost” effect (making the mysteries so cool that no reveal can live up to them) is in the forefront of my mind. My feeling is that instead of dragging them eight books, I should be quick to give answers in future volumes. The things that span eight books as secrets shouldn’t be the ones that you’re wondering at in the first book; they should be the things that, after you begin wondering about them in the seventh book, you can look back to the first book and see the hints. Then you get your answers in the eighth.