It gradually emancipated the slaves and forbade the importation of new slaves into West Virginia. It didn't completely satisfy anyone but compromises seldom do.
From my reading - correct me if I'm wrong - the Willey Amendment freed no slaves when West Virginia became a state. And there were no provisions to free slaves over the age of 21 - they were expected to die in slavery sometime early in the 20th Century.
Children born to slaves after July 4, 1863 would be free but their parents would not be free. Since Lincoln's plan to deal with free black people was to repatriate them to Africa, or somewhere, it seems the free children were to be taken from their slave parents and sent elsewhere. Why do people think the Willey Amendment was good? It sounds cruel to me.
If Lincoln wanted to free the slaves, he should have just asked Congress to pass a constitutional amendment to free the slaves. And not in 1865. He should have done that in 1861 - or even earlier in his political career. That way the war and all the killings could have been avoided.
Creating a new slave state in the union just seems an odd way to free the slaves. Maybe there were important political or economic things going on behind the scenes in the union government that we don't know about.