One of the most frequent criticisms for a book is lack of challenge. One way to avoid this is to make sure that every time your protagonist is within reach of what he or she wants that something gets in the way to prevent it. This builds suspense as well as character engagement. You want your readers rooting for him or her and they’ll be a lot more sympathetic if it’s not easy.
I don’t know about you, but I tend to baby my favorite characters and don’t like for them to suffer or be discouraged. However, that is not how life really is. If the plot proceeds too smoothly, it will make readers roll their eyes and maybe even throw the book across the room (not advisable with an ereader). While a satisfying ending may ultimately see them getting what they want, they should work for it, the harder the better. In good rule to follow is if things are going too well, stop and figure out what could go wrong. Then throw that obstacle in their way.