We investigate the effect of polymer concentration on the diffusion and localization (entropic trapping) of linear polymer chains in a two-dimensional model system of small obstacles and large pores. Three distinct regimes are identified: the entropic trapping regime, a reptation regime where the larger pores are polymer saturated and the untrapped polymers reptate, and finally a crowding regime where intermolecular interactions dominate. In this model system, the entropic trapping, reptation, and crowding mechanisms compete and lead to a characteristic maximum in the diffusion coefficient for intermediate polymer concentrations.