What is the one thing that God holds back from man? Is it rest?

Yes, you are right. In George Herbert's "The Pulley," God pours out an abundance of gifts on humankind but decides to withhold the gift of rest.
God has good reasons for this, which he explains in the poem. First, if he gave humans rest—in other words, if we never got tired—we would "adore" all God's gifts but not God, the giver of the gifts. We would think the gifts came from nature, not from God. We would not need to make space in our lives for God.
God, according to the poem, counts on humans to turn to him when we grow weary. God hopes that "goodness" will bring people to lean on him, but he also knows that this is not always going to be the case. Therefore, he has withheld rest so that, if goodness doesn't drive people to him, exhaustion will.
On some level, this seems odd or counterintuitive. However, in many testimonials, it is the case that it is not until people get so exhausted and feel like they can't go on that they turn to God. It is true too that, if people never got weary, they would think that everything would be in their own power; hence the disregard for turning to or relying on a higher power.