If you're going to have a common currency, then you need to have common banking laws and uniform tax laws--and enforcement of said laws. You'll need a degree of leveling of the social spending in order to get the taxation equalized. This will make some regions more attractive for workers, so you'll need freedom of migration for work purposes. There will be social changes as Polish plumbers turn up in Yorkshire in mass to ruin the wage structure, and as London bank employees start pumping up the real estate prices in nice resort places in eastern Europe.

Isolating the problem to one of currency stability isn't going to be enough...