The main concerns are related to loss of sovereignty..

The EU has its own court , which will enforce laws onto member countries.. Simple things like smacking a child to the death penalty.

The EU is also proposing to have a President. People are unsure, what sort of power he, or the EU will have over the governments of countries.

Lastly, taxation etc, can be dictated by the EU..

So you can see, an ordinary persoon is vary of the power of the EU.