Operating characteristics of quantum dot (QD) lasers are discussed. Carrier-density-dependent internal loss sets an upper limit for operating temperatures and considerably reduces the characteristic temperature. Such a loss also constrains the shallowest potential well depth and the smallest tolerable size of a QD at which the lasing can be attained. In a laser with a single layer of QDs, this loss can strongly limit the output power and cause a rollover of the light-current curve (LCC). Excited-state-mediated capture of carriers from the waveguide into the QD ground-state places a fundamental limitation on ground-state lasing - the output power saturates at high injection currents. The saturation power is controlled by the transition time between the excited- and ground-state in a QD; the longest, cut-off time exists, beyond which no ground-state lasing is possible. Spatial hole burning causes the multimode generation, reduces the total optical power and contributes to nonlinearity of the LCC.