United States of Europe

In principle, there is not much difference between United States of America (as it was intended) and United States of Europe (as it is perceived as of date). 

1. USA was a federal structure in a true sense. Barring money, national security and foreign policy, not much role was envisaged for the central government. However, just like in the case of USA, USE must be wary of tendency of the central government to start hijacking things to itself. Sometimes there are legitimate reasons for doing so, for example:
1. Inter-state issues of national security and policy. For example, establishing of FBI, DEA etc.
2. Creating economic efficiency. For example, inconsistent laws between states creates problems and there is good argument for center to create a unifying law.
2. Other times, central agency takes up power at times of crisis when collaborated and concurrent action is required by all the states together. Naomi Klein in Shock Doctrine, highlights some of the instances where disaster was used to by-pass checks and balances of the system.

Implication

So I don't see any problem when there exists a possibility of politically uniting Europe. If handled properly, it could be successful. However, odds are always stacked against it. Pursuing it at this moment will be a mistake. I am reminded of the quote, friendship always exists among equals. In today's Europe, there is no equality and hence no friendship.

Aside

If you have enough decentralization, then any aggregation at the top level has little or no effect. It is the centralization that starts creating the problem. It belittles the citizen and magnifies the government. 

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