The best comp I can think of with Williams is a sane Michael Beasley.
I think Beasley is a little more skilled, but overall they are similar. Both are kind of tweeners, who dominated in college, partly because of their ability to match up with PFs, and really take advantage of their speed and shooting against the slower competition.
In the NBA though, they really translate more as SFs, which takes away some of their quickness and skill advantage they had in college. I think that is what will keep both of them from being true superstars...but still tremendously productive NBA players.