If we stay in, nothing changes. The Prime Minister and people inside the EU government promise that if the UK stays, there will be compromises, but at the same time they've said there is no leeway on certain rules, which happen to be the ones that people are unhappy about.
The open borders thing works like this; once a country joins the EU, citizens from there can go to live and work in any other country inside the EU with absolutely no restrictions. There is no opt out. Also, official licences and documents from one country must be accepted in another. In practice, this can result in things like my half blind Greek ex using her Greek driving license, which is much, MUCH easier to get then a British one, to drive in the UK. Also, every time a new country joins the EU, there is a massive flood of immigrants from the new countries into the West. And I can't really blame them...why not move to a new country, if you'll have all the same rights as the citizens there, there is nothing to stop you, and the new country is wealthier and has a better social benefit system? The insane refugee policies of certain European countries also makes this something that is not necessarily a good thing at the moment.
The Stay campaign is saying that prices will go up, and the country will lose export revenue, because of lost trade agreements. And I suppose that's possible, and it's the main reason I'm still on the fence. The UK is a big market, though, and I have a gut feeling that most of the countries in the EU will be happy to make agreements that are identical to what they have now.
Basically the whole continent is stuck between the Articles of Confederation and the Constitution, and it's not a very good place to be. And the whole place consists of different nations that have literally been at each other's throats for centuries, and are still racist to each other. I have my doubts that the unification process will go anywhere near as smoothly as the United States' did.
ETA - The fact that the UK is obligated to give a certain amount of money each year to the EU, and gets a lot less in return, is another factor to consider.