Summary:

updated global Reanalysis for the satellite era
fixes known errors in N/N Reanalysis
some improvements in the model and assimilation
improved diagnostics (esp. clouds)
precipitation looks better (still has flaws)
(surface/top-of-atmos.) fluxes on the whole look better
should be used by people affected by known errors in N/N Reanalysis
differences N/N Reanalysis are informative
sensitivity experiment to modest changes in model
rough idea of the analysis error and which variables are "sensitive"