The Commission has even stronger powers in the field of the internal market and competition policy. And there is a long tradition and case law. By these tools the commission has accelerated the liberalisation in member states, controlled the state aids and promoted competition. I am sure the commission will not hesitate to use these competencies in the new member states.
And there are two more incentives: Most of the new member countries are working towards joining the European Monetary Union (EMU). Maastricht criteria and associated institutional reforms required will certainly foster future reforms, and not just those required in Maastricht criteria explicitly.