This article was a bit better than most for Shenzhen- it was at least willing to speculate that a lot of Shenzhen’s advantage now comes from talent and infrastructure. It’s still pretty common for people to attribute it entirely to lower labor costs- which is just not the case.
Shenzhen, like New York is an immigrant city. People come from all over China to get ahead, and get rich. Unlike New York it’s only a bit over 30 years old- and back then it was basically a fishing village. This is important because of the Hukou system: http://en.wikipedia.org/wiki/Hukou_system It’s a bit long to go into (check the link) but basically it ties people to their birthplace and gives them a significant “home field” advantage.
If you are in Shanghai or Beijing and have a local Hukou you have access to the best education system, the highest paid jobs in both the public and private sector, and often a large network of friends and family members in local government. People from Shanghai or Beijing rarely immigrate to Shenzhen because they lose that advantage and are forced to compete with what they would consider the rabble. People from other provinces can rarely compete on even ground with locals- both for legal and cultural reasons. The rivalry is less like US States and more like countries within the EU- but worse.
A “local” in Shanghai may feel perfectly entitled to cut to the front of a line if those people are "Waidiren” (outer province people- 外地人). It’s considered perfectly reasonable for migrant children not to get the same healthcare or schooling. The local dialog about them in every city is a familiar one- basically “damned dirty immigrants taking our jobs and committing crimes”. Needless to say the Central Government is quite keen to keep the provinces deeply prejudiced against each other- because it distracts their attention from the real culprits.
Shenzhen on the other hand is by far the most egalitarian city in China. The city is so young that no one really has an "Uncle" etc. in local government willing to “investigate” competitors or send some easy government contracts their way. There is little difference between those few born in Shenzhen and those who came a few years ago. There is no local dialect that is used to subtly determine who’s “local”- everyone basically speaks Mandarin.
So for people in relatively poor provinces (Hunan, Hubei etc), with brains and education but little in the way of prospects due to their Hukou, Shenzhen represents the best possible opportunity to compete in a first tier city almost purely on merit. Success here is based largely on hustle, brains and hard work- while in other cities at least 50% is simple corruption (well placed relatives in banking and government). Most Chinese would place the number even higher.
So Shenzhen gets a lot of China’s best and brightest, but also those who are inherently ambitious- because they were willing to leave their hometowns and family (a much bigger deal here). A huge amount of the slow grinding machinery of legacy corruption does not exist here (massive numbers of bureaucrats given comfortable jobs doing basically nothing as a form of social welfare). It happens occasional sure- but not to the point that it does in other cities where merit and hard work is almost meaningless next to the right connections.
The result is a giant magnet for talent and a massive, well funded playing field where that talent competes with significant rewards reserved for the most skilled, clever and hardworking.
There are a lot of incredibly smart highly motivated people here- and that, more than just simple labor costs is responsible for Shenzhen’s market position.