You don't know Jack.
And it is fairly debatable about whether you want to or not.
First, let's get the obvious out of the way. The guy is too small. At 5' 9" (generously) he comes to a team that won't draft elite safety help if their lives depended on it, and is guaranteed to be mismatched 9 times out of 10 in one on one coverages. I love D-Will, but I remember him being burned a lot in one on one scenarios against precise passers. He was at his best when the opposing QB was screwing up, where his tenaciousness made teams pay for their mistakes.
Which is something you see in J-Will as well. Tenaciousness. Fire. Competitiveness. You WILL see him in the mix at KR/PR. You WILL see him fly down the field with some of the best timed speed in the draft. You WILL see his willingness to work and his great team attitude. You WILL see a combination of strength and speed that is rare in a CB. And you WILL see a player who will never give up.
In fact, I would fairly say that, of all the words you might use to describe J-Will, WILL might be the best one. Nothing has or will be handed to him. He knows this and he understands it. What he lacks in height he makes up for in an unheralded devotion to filmwork and study. He knows tendencies and he doesn't bite on misdirection, while showing tremendous discipline in not relying on his speed to save him. And he will be entering a DB environment in Denver where nothing less is expected. He has a tough learning curve ahead of him, but he's tackled these kinds of things before, and where he looks like he isn't up to the task, he WILLS himself through with a combination of dedication, experience and tenacity.
His range is excellent, and he knows how to get his hands on the ball, though he was allowed to zone frequently in his college scheme and thus would attack the ball. But as a nickle back, he may be able to combine the best of what he does with the best that Slowik will require. Like Darrent, he may be asked (or prefer) to play with some cushion and rely on smart safety zoneplay behind him to read the route and the play, which are two areas he excels at. It is to be expected that he may struggle with man coverage, although it should also be noted that he possesses many of the traits needed for good man coverage ability: he moves his hips well and has great anticipation, and accelerates with good burst. He reads receivers well, though he can be caught with good double moves, and he needs to get better with his hands, since he doesn't have the size to stay with a receiver off the line, and is not good at rerouting the receiver.
Regarding the pick itself, Denver has shown that they are not interested in leaving much to chance. They KNOW that Abdullah, Foxworth and Paymah will be free agents, and they know that it may even be in the teams best interests to trade any of those players. There isn't time to lose in insuring the defensive backfield, and with only weak options at safety, they looked to get the kind of player that can work hard and make other's expendable in his wake. Foxy and Paymah will both have to EARN their free agent contracts from Denver next year, and this pick ensures that.