What I like about Watt is that he is ... instinctive. He tracks the QB or RB and makes the tackle, rather than blow past the guy. Also, looking at his film, he is damn near skinny. He can add 10 lbs. of muscle and retain his quickness.
I don't scout football players, but I have scouted baseball players. When scouts look at baseball players, one thing they look at is body type. Is the guy "maxed out" in terms of his muscle and strength, or can he get stronger and faster? If a guy is "maxed out," that weighs against his projection. Great, the kid dominates his peers, but what about getting bigger and stronger to go against adults?
Watt reminds me a lot of a hitter who is damn good, but is not fully filled out. He is going to get better, and produce against the big boys.