Arrogance is a weakness in itself, Sasuke screws himself over this way too *cough, kagesummit* lol but Neji is far more arrogant too, he believes he is above everyone in the competition.
The reason that Neji always defeated Lee is because they were always sparring, Lee would never use his strongest (and forbidden) Taijutsu on a comerade no matter what he had to prove. We both know that the Gates opening and Lotus techs. would put Neji six feet under.
Btw, having a "Jaw of glass" is just an expression, it means Neji can't take a hit well. Sasuke gets punched in the jaw all the time and is sent flying into a cliffside, but doesn't tap out.
Gaara is another example of someone with a jaw of glass, he is very rarely hit, but when it does happen, it ould cost him the fight, yknow?