WHAT ARE THE MERITS AND DEMERITS OF DEREGULATION?

The case for deregulation is both an economic argument, and an ideological one. 
Advocates of deregulation will state a positive economic case for reducing the burden of regulations on businesses and individuals.  However these advocates will also likely define themselves as somewhat libertarian in thinking, and have a natural preference for lower interference by government.
The argument in favour of deregulation is the theory that the reduction of interference by government has a positive effect on the ability of business to flourish.  For example a business that doesn't have to abide by lengthy and complicated employment law can more easily hire employees.  Or a business that didn't have to comply with environmental regulation could increase their profit margins.  This in turn should enable a more dynamic economy with increased job creation and stronger economic performance. 
Deregulation advocates would also point to heavily regulated economies, particularly in socialist countries like Venezuela, where tight controls on economic activity are symptomatic of a wider economic failure.
The case against deregulation revolves around the idea that market forces alone cannot be relied on to produce outcomes that benefit the overall population.  The government therefore sometimes needs to intervene in the market, by way of regulation, to ensure the health of the economy and the overall prosperity of the country.
One of the most obvious arguments against the deregulation of an industry would be the global financial crisis of 2008. While the banking industry was heavily deregulated throughout the late 20th Century, the industry eventually ran in to severe financial difficulty as a result of overly risky behaviour and negligence.  A regulated banking industry would in theory have been protected against these excesses and wouldn't have caused the financial deficit, which eventually fell to the taxpayer.
The extent to which deregulation can be said to be a success ultimately depends on perspective.  While environmental deregulation may allow for economic growth and benefit business, it could lead to practices that are hazardous to wildlife and long term damage to the planet.
 
 
 

Deregulation can have some good points. It removes a "one size fits all" form of governmental management and allows individuals and businesses greater autonomy in their dealings and resource management. Deregulation also saves the government money, as one has to pay for regulators.  Also, sometimes there can be biases in governmental regulations which benefit one group over another.
Deregulation can also be a bad thing. The Gilded Age proved that businesses can operate unfairly without regulation. They can sell the public dangerous goods and they can inflate their stock prices through fraud. Regulations are needed to manage natural resources. If the public depends on clean air and water, then there should be a public organization that defends these basic resources. A lack of regulation leads to unfair exploitation of resources.