The sacrificial perogative of the kings of Israel Why was the sacrifice offered by Saul at Gilgal rejected by God while obviously the sacrifices of David and Solomon at a later time were evidently accepted? Did it have anything to do with the death of Samuel?
I would add to the above explanation that Saul was offering a sacrifice while being in rebellion to the command to kill everyone and destroy the spoil.
1Sa 15:3  Now go and smite Amalek, and utterly destroy all that they have, and spare them not; but slay both man and woman, infant and suckling, ox and sheep, camel and ass.' 
This should be a warning to us that we can't offer sacrifice while in rebellion and expect it to be accepted.