I like to write an entire series ahead of time, which allows me to plan things well in advance. The significance of certain details in the first book aren't revealed until much later in the series. I don't know that I've ever taken a count, but it's my hope that readers will find things in later books and remember, "Oh, that's what that meant!"
Of course, this only works in moderation. If you keep overloading readers with references to subtle details in earlier books, they will eventually stop reading you. I've been told one of the strengths of my writing is that it is involving, but approachable. You don't have to learn a new fantasy language to figure out what's going on, or consult a massive concordance or keep doing Wikipedia searches every few paragraphs. I know there are readers who actually prefer that level involvement with their books, and I think people who enjoy that will find much in my work to keep them busy. But, at it's core, I'm just trying to tell a good story.
I'm not Ernest Hemingway or Frank Herbert. They were titanic authors who wrote timeless stories. The mere presence of their name on a work of literature is enough to attract an audience. I'm confident about my work, but I'm not going to impose too much on my readers. In other words, I assume that I have to earn their interest; it's not just a given like one of the above-mentioned great writers. I think the worst thing a writer can do is fall too in love with their own work. It's important that a writer enjoy what they write, and write what they enjoy...but equally important is to remember you have to earn a reader's trust and respect with your writing. Make them work too hard for it, and you take the fun out of it. People don't mind reading James Thurber or Franz Kafka with a concordance or online encyclopedia nearby. They won't be so patient with a less known writer like me.
On the other hand, there are a few things in my books that I keep hidden from most people. I do this for fun, and also to see if anyone picks up on them. I'll admit that it will be quite a rush to see people try to figure out what a certain character's name means, or if there is any significance to seemingly innocuous details. Some of these secrets I may share when the books come out. Others, I may keep to myself and a small circle for my friends.
Why? Because it's fun, that's why.