Free movement of goods and services. Great idea, even though competition is often unpleasant for those who are under its pressure. However, being protected against competition by trade barriers is bad for everybody.
Free movement of capital. Great idea, too. Imagine world where you encounter endless troubles when you wish to invest abroad: capital controls, special permits and that sort of nuisance. No, you don't want to return back to the bad days of a non-convertible currency and multiple exchange rates.
Free movement of labour force. Good or bad, it depends. You may like it or not, depending on whether you are an employer or an employee, whether you live in the Western or in the Eastern Europe, etc. Economic theory says free movement of people is good, because everybody would be better off in the end. In practice, can you imagine all the poor and huddled masses from Asia, Africa and Eastern Europe coming to the EU in pursuit of their happiness? Well, hardly.
Welfare tourism. Bad. There are countries, which are generous to the point of absurdity. Entitlements in most welfare states are expensive enough when provided for the country's own citizens. Add millions of immigrants, who usually do not contribute much to the system – and the budget balance would sink deeply to red numbers before too long.
Welfare state. Mostly bad. Nobody wants to leave the weakest and the most vulnerable to starve, but the modern definition of who is needy is dangerously broad. A state that punishes the productive and working and rewards the unproductive and lazy devours its own future.
United States of Europe. Heaven forbid. Concentration of power is dangerous in any case. In the case of the European Union, which lacks democratic checks and balances, more integration would lead to the inception of a totalitarian regime. The USE would quickly turn into the EUSSR. According to some opinions, it has already happened.
Tax competition. Excellent. A number of EU dignitaries keep complaining about it, since the competition among tax jurisdictions prevents income tax rates from being raised to stratospheric heights across the continent. As soon as the United States of Europe (a.k.a. European Union of Soviet Socialist Republics) would emerge, a federal European tax would be introduced. Unlike the USA, the USE would lack the necessary systemic checks and balances; soon we would end up where Sweden was in the late 1970's. No, thanks. (Note that Sweden has a lower corporate income tax rate than the USA—and it has been known as a tax hell!)
EU enlargement. Not good. Frankly speaking, the EU is already now too big. Understandably, unelected bureaucrats in Brussels would like to enhance their powers over the dependent territories. However, is it a good idea for