It depends from who. A big one, in my opinion, is the loss of power/sovereignty, in the sense that countries of the EU have to follow the European Directives th

It depends from who. A big one, in my opinion, is the loss of power/sovereignty, in the sense that countries of the EU have to follow the European Directives that are made in Bruxelles, and that if they don't, they have to pay huge penalty fees. Another one is that, on a lot of different matters and areas, different countries just cannot obey to the same laws, and their history, language, and way of thinking are just fundamentally different.