Johnson is a big man with a ton of natural talent. He has an incredible burst off the edge of the line, and he displays excellent use of his hands and leverage much of the time. Even if he doesn't manage to take down a runner or quarterback, he'll push the pocket and disrupt the play. He also stands up well against the run, collapsing running lanes and forcing the runner to other defenders. Also, as big as he is already, his frame has the room to increase his weight possibly as high as 290 pounds without losing ability; a frightening thought given his already considerable power game.
Johnson is as raw as could be, and could just as easily be a tremendous bust as he could tap his potential. Some will doubt his results from last season, saying it was a product of playing across from Quentin Moses, an incredible talent in his own right. He also has a tendency to wear down quicker than you'd like to see, but that's inexperience as much as anything, and it'll get better with time.
A high-risk, high-reward prospect, Johnson will intrigue many teams, especially teams like Miami that run a hybrid defense. He's already a top prospect, and the better he does at the Combine, the higher he goes. Expect him to go in the upper half of the first round, with a good chance at cracking the top ten.