On paper at least, the UK and Denmark do have special opt-outs from the Euro while all the other EU states are legally bound to join it. In reality it's likely that several of the non-Eurozone states will simply never join it and there's nothing to ensure that they ever do.
The point about other European countries is well taken and I believe there have been talks to get some countries out of the Euro zone. In part this is a big deal because countries like Germany don't want to prop up countries that are in financial difficulty. They have no controls for dealing with these problems and the result is that you need cash infusions from the EU to keep it all swimming.
But that's really the point, isn't it? How much of a "Union" do you want? It even gets to the heart of why some people want Briton to leave the EU.