Young trees are so impressionable. Too much water can damage their roots, or cause them to disperse too shallowly. Improper pruning can disfigure their branch structure, and ultimately compromise structural integrity. Improper staking to keep them stable can actually interfere with development of stabilizing roots, and interfere with trunk development.
Newly planted trees will of course want to be watered from spring to autumn for at least the first year, and more likely for a few years. Those that will eventually be less reliant on watering as they mature are actually the most demanding while young, because their confined roots are not yet adequately dispersed for self sufficiency. The problem is that too much water can keep lower soil too saturated for new roots to disperse into. This causes roots to instead disperse closer to the surface of the soil, which is not only unhealthy for the trees, but puts the roots closer to pavement, other plants and any other features that they can eventually damage as they grow.
Many young trees should be pruned as they grow to eliminate structural problems, and to instead promote good branch structure. The problem is that improper pruning can actually cause structural problems that will be with the victimized trees for the rest of their lives. Pruning should leave no stubs that will take longer to compartmentalize (heal), or that might produce vigorous but weakly attached new stems.
Stakes are unfortunately necessary to stabilize new trees. The problem is that the trees can become so reliant on stakes for support that they do not develop enough trunk strength to support their canopy without stakes. That is why trees should be tied loosely enough to their stakes to be able to move at least a little in a breeze. Nursery stakes (that trees are bound to for a straight trunk in the nursery) should be removed when sturdier stakes are added.
New trees are naturally a bit more distressed than mature trees that have settled into their environment. They are consequently more susceptible to disease and insect infestation.