We follow the bright, highly energetic afterglow of Swift-discovered GRB 080721 at z=2.591 out to 36 days or 3e6 s since the trigger in the optical and X-ray bands. We do not detect a break in the late-time light curve inferring a limit on the opening angle of theta_j >= 7.3 deg and setting tight constraints on the total energy budget of the burst of E_gamma >= 9.9e51 erg within the fireball model. To obey the fireball model closure relations the GRB jet must be expanding into a homogeneous surrounding medium and likely lies behind a significant column of dust. The energy constraint we derive can be used as observational input for models of the progenitors of long gamma-ray bursts: we discuss how such high collimation-corrected energies could be accommodated with certain parameters of the standard massive star core-collapse models. We can, however, most probably rule out a magnetar progenitor for this GRB which would require 100% efficiency to reach the observed total energy.
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