His frame is excellent for a defensive lineman. Because he was playing 4-3 DE in college (and because players don't usually max out their body size in college) he's a little light for a 3-4 lineman. However, his length, strength and a frame that looks like it can take more weight without hurting his athleticism still make him a good prospect as a 3-4 DE. Durability is not a concern here either. He played every game in his 3 years as Wisconsin, including work on special teams and a few games as a TE.
This is probably the best part of J.J. Watt's game. He has quick violent hands that he can use to win at the point of attack. He also has multiple moves that he can employ and can use moves early to set up some other moves later on. He can also use his power to do some bull rushes, but the key is still hand placement. Once he gets off his block, his closing burst is a strength given his size. His ability to harass a passer in the back field with his hands (he's good at knocking down passed), his speed and his strength is his top skill.
Hands are still his best weapon in this area. If he can get off the ball quickly and attack the linemen then he can get penetration and cause disruption. If not, then he can sometimes get overpowered and will struggle with double teams. His size and strength still help him to hold the point of attack, but likely will not make many plays in the backfield if he isn't the fastest off the ball. Because he's only coming out as a Junior, still has room in his frame to grow and has such a high aptitude there is a definite possibility that any issues he has in this area can be fixed once he's under the tutelage of NFL position and strength coaches.
Extremely active in pursuit. High motor is a term that's used a lot with him and it's possible that his is higher than anyone in this draft class. That motor allows him to be a player that "always seems to be around the football". His pursuit skills help him to pick up fumbled football or help with a downfield tackle of a RB or TE. He's not a technique tackler and can miss some from time to time, but he's aggressive, hits with power, has good strip techniques and can jar a ball loose.
He's an every down type of player. In passing situations you can slide him inside and line him up against guards. On running plays you can play him as a 5 technique or possibly have him on the edge since his pursuit skills are good enough that you won't miss losing an extra LB.
Definitely worthy of a 1st round pick. The things keeping him out of being him a lock for the top 10 are skills and conditioning that many scouts would say could be developed at the next level. His strengths are good enough to where he could start right away with most systems. He's probably a better draft fit for the 3-4 since he'll probably never be a 10+ sack player in the 4-3 and that's what teams look for in the 1st round. In the 3-4 he'll have work to do, but his quick hands, versatility, motor and power will carry him until that work is done.
There is one issue however. A.J. Smith watched him on his Pro Day. Now, some might point to that as an indication that the Chargers want him, but we should know better. For years Smith has used subterfuge to keep everyone off balance. Players linked to the Chargers never came to San Diego and ones that had never heard from Charger scouts were drafted often. Only recently broke from tradition when he went gaga over Ryan Mathews. And how did that go? The NFL conspired to force him to trade up and lose his 2nd round draft pick in order to get the young running back. A.J. has likely learned his lesson. So, that might be the death knell for Watt's chances of becoming a Charger.