Why would European countries that are not part of the European Union be at a disadvantage?

Answer 1

If you live in a Country that is not part of the EU, you cannot travel freely around countries that are. Also, countries not part of it can't trade goods as easily.

Answer 2

Switzerland or Norway are in no disadvantage, they have strong and rich economies and autonomy from the often silly european union regulations and bureaucracy. However, countries like Greece, Portugal or the ex communist states, do benefit from substantial funding of their infrastructure, social system etc.

Answer 3

It's not true that you can't travel easily or trade, look at the countries that are not officially part of EU like Switzerland, or Norway, they are in better position than the ones that recently came in EU. EU has it's political, cultural and economic implications. I am not sure that new members will much benefit, but at least for some countries (like ex-yu republics) that will be good in political sense, like a guarantee that there will be no more war. Plus EU is in it's biggest crisis these days (problem with constitution and disagreements between members, like: Turkey, yes or no in EU, etc, etc).