Workers' Rights
Of course, the rights of employees, including conditions of employment and minimum standards for the work environment, must be part of the Common Market. If not for anything else, then it is at the very least part of the level playground rules behind any free competition market. Is it? Well, not necessarily, but in EU it must be, because the voters in general expect it. There is no reason to question that situation, because it is a democratic requirement and it has generally served most member countries well, for decades. It has also served for removing tensions in the internal market, which is something we should all be content with. The problem of course is with imports and what work conditions these imported products are produced under. That one is still under discussion and it cannot be expected that EU can or will solve any feasible workers' rights problems all over the World outside EU.
With minimum wage, there is a peculiar problem that shows some of the cultural differences within the EU that are basically irresolvable unless the EU gets power to supress any regional and national differences with matters that concerns the Common Market.