Saul destroyed an enemy, but failed to utterly destroy them, as God had commanded him. Instead, he saved the choice animals and offered them as a sacrifice to God.
"And Samuel said, Hath the Lord as great delight in burnt offerings and sacrifices, as in obeying the voice of the Lord? Behold, to obey is better than sacrifice, and to hearken than the fat of rams." (1 Sam 15:22)
The law of sacrifice is NOT selecting what you will sacrifice to the Lord. It is not imaging up what you think would be good for you to do and then doing it. It is asking God what he wants you to do, and then doing it.
Sometimes it is easy for us to get carried away imaging what would be pleasing to God. We end up stretching ourselves in vain, while ignoring the real soul-burning offering that God has asked of us. Most times, this harrowing experience is very personal and possibly even invisible to those around you (though sometimes it is public).
God is not pleased by the quantity of what we give. He is pleased by how hard it is for us. The sacrifice is not what we have given out but what we have given up in ourselves by yielding to God.
"Behold, I have seen your sacrifices, and will forgive all your sins; I have seen your sacrifices in obedience to that which I have told you." (D&C 132:50)
Every day we are presented with altars and promptings that give us the opportunity to offer up some part of our will for a sweet savor to God. Do we take these often externally quiet opportunities? Or, do we ignore them and focus on the outward motions that for all their pomp do not yield progress with God?