Mike Williams had a terrific rookie season and showed that he could be a great wide receiver in the NFL. He's a good route runner who uses body and head fakes to create separation, he knows how to catch a football at his highest point and is an elusive runner after the catch. The fact that he broke team records as a rookie receiver makes him look like a future star, but there's one thing he needs to stop doing: dropping the ball.
Mike Williams had 6 drops last season according to Football Outsiders' game charting data, and 11 according to Pro Football Focus. Regardless of who is correct, that's simply too many drops for a good wide receiver. Looking at the game tape, the drops happened in different situations: they happened on short, medium and long passes, and they happened on catches in traffic and in the (relatively) open field. None of them happened because Mike Williams let the ball into his body, as he consistently catches the ball with his hands. The one thing you do see on the tape, though, is that the rookie takes his eyes off the ball just a little early, either looking to set up his next move or looking to see where the defender that's going to hit him is coming from. These are simple mental issues that he should be able to eliminate.
For next season, Mike Williams needs to concentrate on catching every ball he can. If he does that, he's one step closer to being a complete wide receiver.