I think the thing I have liked the most is that he clearly understands his role in the Celtic offensive sets. When Stevens calls for a multiple screen, or multiple step set, Williams knows his part.
He doesn't get lost on offense. He is not a step slow on his actions or cuts. He seems to have a really good feel already for the sets Stevens runs. That's promising for a young athletic big. Normally it takes a year or two for them to run their part of the offense as efficiently as Williams is.
On defense, when he switches onto opposing guards, he has been pretty good, but when he is off-ball, I've seen him be out of position or a step slow.