Key phenomena controlling the transport barrier in H-mode plasmas on the Alcator C-Mod tokamak are discussed. Type I edge localized modes (ELMs) are not observed. Instead the particle transport barrier is continuously degraded giving rise to `enhanced ' operation which avoids the deleterious impurity accumulation observed in `ELM-free' H-modes. The dependence of operational regime on plasma parameters suggests that the instability responsible for barrier degradation is enhanced by higher pressure and longer field-line connection. Detailed analysis of the magnetic structure of fluctuations associated with the different H-mode regimes shows that these field-aligned fluctuations propagate with large perpendicular velocities believed to represent the large velocity shear responsible for barrier formation. In the enhanced regime the fluctuations are generally more incoherent, but have not otherwise yet shown a specific distinctive character.