More and more I see the argument that the EU should be turned into a free trade area, and I thought I should try to create a short reply.
"But we signed up to a free trade area!"
This is also argued with "common market", with it clear that a free trade area (like NAFTA, for example) is meant. But there's a big difference between a free trade area and a common market (a.k.a "single/internal market").
A free trade area simply reduces tariffs and import quotas between countries - this was the aim of the European Free Trade Area,* which the UK helped found in opposition to the EEC, and then left to join the EEC. FTAs don't cover the free movement of goods, people and services that a common market consists of.
The issue with free trade areas that they only reduce tariffs between members, so members can have different tariffs with third countries. This means that some goods might try to get into the free trade area via FTA country A (which has lower external tariffs) before moving on to country B (which has high external tariffs). This lack of control means that some countries prefer a customs union, which is like a FTA but with common external tariffs.
A common market is much more than a FTA because it not only deals with tariffs and import quotas, but also deals with the free movement of goods, services, establishment (freedom to set up businesses throughout the common market), capital and people. This requires some restrictions on the law-making of member states (they can't legislate for import quotas, discriminate against goods from outside the country with different tax rates, etc.), and requires common law making so goods, etc., can move freely without being blocked by different technical standards. This common law making and standards means that there would need to be a common court and law-making institutions in any case. That's why the EU needs to have institutions, but NAFTA doesn't.
Leaving a common market means that a country and its citizens would loose free movement rights. While it's true that there will still be trade between the EU and ex-members (the EU has FTA agreements with Chile, Mexico, South Africa, etc., so I don't see why an ex-member wouldn't get one as well), without common laws, the ex-member would move away from the common standards of the common market, and more and more non-tariff barriers would block trade. Citizens of the ex-member country would also loose their free movement rights.
Having a common market also means that there needs to be a common trade policy with third countries, and the free movement of persons creates pressure for there to beco-operation and common rules on cross-border crime, but foreign affairs and justice and home affairs are other debates.
Since the EEC was aimed at creating an common market, joining countries never signed up for a free trade area.
*Confusingly, the EFTA is now a single market because its member countries are linked up to the EU's single market via the European Economic Area (or via 120 bilateral agreements in the case of Switzerland).