What made the Qin army so powerful? There are many reasons for this.
1. The establishment of the military code which was strictly enforced in the army. It had two parts: reward the valiant soldiers and punish the cowards, no matter his rank or birth. This had serious consequences in the armies of Qin. At a time when armies grew larger, a disproportionate number of troops were drafted from the peasants (who were the so-called "wild" men, or Ye Ren, people living in the wilderness with no political power, unlike the countrymen or Guo Ren who could trace their ancestry to the nobility). These troops formed the bulk of the army. When one is able to motivate these men, the army would become truly powerful.
2. The Qin culture is warlike and aggressive. There is no other state that would witness 10000+ men being promoted after every major large scale battle. This happened for the Qin armies and her soldiers became fearless. Families of soldiers were reputed to be An Nan Le Si. They find peace to be hard and death to be celebratory. They would celebrate when there's war against the other states. It is one of the few instances in Chinese culture in which the warrior culture is epitomized and made into the state culture.
3. The Qin army is not only well motivated, but well armed. Millions of men, criminals and prisoners of war were enslaved in huge arsenals manufacturing weapons. The Qin, lacking significant iron, copper or tin deposits, were also very careful with their metal and set up an official post to ensure that no metal go to waste. The Qin culture also prevented the use of metals to make instruments for rituals or musical instruments. All would go for weapons or agricultural tools. The Qin get their ritual pieces by defeating other states.
4. The Qin army rarely have to worry about food when it comes to attacking other states. The Qin agriculture was extremely robust as supported by Shang Yang's reforms of emphasis on agriculture and heavy taxes for the merchants. The regions within the Pass (Guan Nei) was made fertile and productive through wise and long term investments in water works, and even the treacherous Zheng Guo was made to apply his genius in making the Canal of Zheng Guo which helped to create vast tracts of arable land to feed the armies of Qin. A well fed army fights better than a half-starving army.
What is unique about the Qin army?
1. The Qin army is the largest in the 7 states. Her armies are the most fearless, most aggressive and though it has suffered some huge defeats, their spirit remained indomitable and they helped the King Zheng of Qin to unify China.
2. The Qin army is known for their crossbowmen. Right...I have been influenced too much by hero, but from the terracotta army we see that there's an emphasis in ranged units with both archers and crossbowmen. Crossbows are manufactured in vast numbers in the manufacturies and were reputed to have replaceable parts.
3. The Qin army is well organized with a clear distinct hierarchy. A unit leader is responsible for his men, and his men are responsible to him. In any given moment, a Qin unit is entirely independent as he sought to kill the enemy and at the same time cohesive as breaking any military law means death. It is this willing to advance without fear and cohesiveness as an army that makes the Qin army powerful.
4. The Qin army is led by experienced generals. Bai Qi, Wang Jian, Wang Ben, Meng Tian, these are all generals who have fought campaigns after campaigns. Despite some reverses suffered by some of these generals, the soldiers trust their generals to carry them through for their reputation. The other states had no famous generals or even if they had, their rulers foolishly got rid of them.