The lectionary gives us great passages that we might not otherwise read. The Psalm is a long litany of all the things God has done for God's people. God took care of the wandering families of Abraham. God sent Joseph into Egypt so that they, and everyone else in the region, might have food during famine and live. God made them a great number while they were in Egypt. God set things up so they would leave Egypt. God took care of them in the wilderness. God led them into the land of Canaan. God remembered his promises to Abraham and kept them. The Psalm is a long read. The form reflects the message: reading all the ways God has blessed us should take a while because God's deeds are mighty and many. God's blessings are overflowing. The Psalm goes on for a while and gets detailed because God's blessings go on.
The lectionary is also sometimes criticized for cutting out parts of the scripture. Today's Psalm is long, but it only had three more verses. These verses express how joyful the people were, and the expectation that they would observe God's laws when they got there. Through the entire Psalm there is no criticism of the people (criticism of the Egyptians, yes, but not of God's people). This misses the theme of the other passages in the lectionary. Numbers talks about the people's rebellion in the wilderness, and I Corinthians talks about us learning from their mistakes. The lectionary is good for pushing us to read more scripture, but it's ok to break the lectoinary rules and read more. It's ok to ask yourself Why the lectioary left something out. It's ok to go against the lectionary's theme. The lectionary started us reading Psalm 105: Yay, lectionary! but it's also ok for us to finish it. Sometimes we find joy there: "So he brought his people out with joy,his chosen ones with singing."