Why is Rock Lee unable to use ninjutsu or genjutsu?
He simply has no talent whatsoever for it. However, since he decided to be a great ninja nonetheless, he had to excel at Taijutsu.
Taijutsu - it was the only bright spot for Lee who was told unequivocally that he had no talent for being a shinobi. 1
Ordinarily, the inability to perform both Ninjutsu and Genjutsu would mean that life as a ninja would be impossible. But in Lee's case, his closeness to Gai as his Sensei, made him somewhat 'inherit' his hard-working resolve as well as his training habits, which combined (also with Gai interest in Lee) have allowed him to fulfill his dream.
Lee's case is similar to Naruto's, in that they both achieve greatness through hard-work, rather than natural talent.
Some people are naturally gifted toward some types of technique, like Neji or Sasuke, while others may never have any talent and thus not be fit to become a ninja. Lee's case is the latter, but his determination was what drove him to not give up, becoming an excellent ninja at Taijutsu.
He differs from a regular person, most likely, in sheer will, since (as I've said before) not every person is fit to be a ninja.
In the Naruto universe, only a few are born with the talents of Ninjutsu and Genjutsu. You must have noticed that not all in the Hidden Leaf village are ninjas as they don't possess those skills to manipulate their and others chakra. Hidden Leaf in the Shinobi village of the Fire Country and yet not all are ninjas. In the Shinobi Allied Forces, around 30,000 shonobis were there which included ninjas from all 5 nations' villages. So you get the idea that among a population of millions only 30,000 were fit to be ninjas.