Gratitude is the produce of seeing and experiencing past grace. In Exodus 20:2-3 God says: 'I am the LORD your God, who brought you out of the land of Egypt, out of the house of bondage. You shall have no other gods before Me.' God is encouraging the Israelites to obey Him based on grace He has already given, the deliverance from the house of bondage.
Hope is the produce of seeing and experiencing promised grace. In Deuteronomy 8:6-7 the Lord motivates obedience by reminding the Israelites of the good land He has promised to them. 'Therefore you shall keep the commandments of the LORD your God, to walk in His ways and to fear Him. For the LORD your God is bringing you into a good land, a land of brooks of water, of fountains and springs, that flow out of valleys and hills'
It's then that I looked up what Piper had to say and found this:
Piper's argument is this: God is more glorified when the Christan is motivated by hope to obedience because hope requires more relying faith than the faith required in gratitude. Gratitude requires a simple accepting faith. The faith we have in past events, that have happened by God's grace, is a not relying faith it is only an accepting faith. Something like this: 'I thank you God that you did this or that for me and I believe that it was for my good and your glory.'
Hope say things to God like: 'I thank you God that I will be like you (1 John 3:2)' Saying this requires more faith than that of gratitude because I'm yet to experience the grace for which I'm trusting God for. I'm simply trusting that I will be like Him. This trusting is based upon God's faithfulness displayed in promised grace.
If then hope is the main motive for Christian obedience, is hope the main motive for other types of obedience? How do you get a child to eats it dinner? You would never say to that child 'Don't you remember what I got you for Christmas?' You would say something like 'Remember that Christmas is coming soon. I've got presents for you.' By using the second statement the child is more likely to eat his or her dinner and the parent is seen to be greater because the child has expressed a trust in the promise of the parent.
So what God is doing in asking us to be mainly motivated to obedience by hope is firstly, glorifying Himself and secondly, setting our hearts on a future enjoyment of His goodness. Gratitude makes us thankful and appreciative people but gratitude lacks vision and courage. Hope makes us an expectant, lively, excited, people to more of a God-glorifying degree.