During our prayers we praise God constantly, but in the Kaddish we excel in expressing our praise to God, more than in any other prayer.
We say (or respond): yehe shemeh rabba... "May His Great name be blessed, praised, glorified, exalted, extolled, honored, adored, and lauded".
The rabbis were not impressed with the abundance of synonyms. On the contrary. They emphasized that the real praise comes in the following phrase: le'ela min kol birkhata.... "May His name be praised... beyond any blessing and praise... that could possibly be said in this world". The greatness of the Kaddish is that it makes us realize that no matter how many words we would use, we will be always short of grasping and acknowledging all He is, and even all He does for us. No matter how much we praise Him, for those of His actions that we are able to see, we admit that the full scope of His goodness is completely beyond our reach.
Let me illustrate: A five years old child can not not possibly realize all what his father or mother do for him. He might say 'thank you' for an extra candy, but a candy is only a small portion of all the parents do for their child: Love, care, attention, food, clothes, health, education, comfort, etc, etc, etc.
And we are like a child, that perceives only the 'candies', a small portion of all He does for us. The bigger picture of God's kindness, is hidden.
The Kaddish or our response to the Kaddish, is the opportunity to express that if we don't praise Him more, it is not because we are ungrateful, but just because we are limited to perceive all He does for us.