The biggest switch is between the first reading and the second (which is why I try not to review books until I've read them twice).
Reading at a more leisurely pace, you get to notice more. (such as technique and how it's used; once you're no longer insatiably curious about the information, you can notice the info-dumping), but what you notice above all else is clues -- or lack thereof. Because you know the ending, many, many, many things should be fraught with dramatic irony. A character's plans don't take into consideration several facts to be revealed later. A character hysterically sobbing over a death is not only grief-stricken but consumed with guilt. The obvious villain slipped in a few clues that he was not as evil as he seemed earlier than the great revelation.
Sometimes the books are better because the dramatic irony deepens them. Sometimes they're just about the same because what they lost in suspense they made up in irony. And sometimes they just go flat because there was nothing to them but the suspense.