  We show that the centrality and system-size dependence of elliptic flow
measured at RHIC are fully described by a simple model based on eccentricity
scaling and incomplete thermalization. We argue that the elliptic flow is at
least 25% below the (ideal) ``hydrodynamic limit'', even for the most central
Au-Au collisions. This lack of perfect equilibration allows for estimates of
the effective parton cross section in the Quark-Gluon Plasma and of its
viscosity to entropy density ratio. We also show how the initial conditions
affect the transport coefficients and thermodynamic quantities extracted from
the data, in particular the viscosity and the speed of sound.
