Edge transport barriers (ETBs) in tokamak plasmas accompany transitions from low confinement (L-mode) to high confinement (H-mode) and exhibit large density and temperature gradients in a narrow pedestal region near the last closed flux surface (LCFS). Because tokamak energy confinement depends strongly on the boundary condition imposed by the edge plasma pressure, one desires a predictive capability for the pedestal on a future tokamak. On Alcator C-Mod, significant contributions to ETB studies were made possible with edge Thomson scattering (ETS), which measures profiles of electron temperature (20 [leq] Te[eV] [leq] 800) and density (0.3 [leq] ne[10^20m^-3] [leq] 5) with 1.3-mm spatial resolution near the LCFS. Profiles of Te, ne, and pe = neTe are fitted with a parameterized function, revealing typical pedestal widths [delta] of 2-6mm, with [delta]Te [geq] [delta]ne , on average. Pedestals are examined to determine existence criteria for the enhanced D[alpha] (EDA) H-mode. A feature that distinguishes this regime is a quasi-coherent mode (QCM) near the LCFS. The presence or absence of the QCM is related to edge conditions, in particular density, temperature and safety factor q. Results are consistent with higher values of both q and collisionality [nu]* giving the EDA regime. Further evidence suggests that increased abs([nabla]pe) may favor the QCM; thus EDA may have relevance to low-[nu]* reactor regimes, should sufficient edge pressure gradient exist.