Why can't a human regenerate limbs like some other species?
After salamanders (or certain other amphibians) lose a limb, the epidermis migrates to cover the wound, forming a structure known as the apical epidermal cap. The underlying "stump" then generates a de-differentiated mass of cells from normal tissue. In most cases, these cells will express genes that are vital to the proper formation of limbs, such as HOX genes. This "lump" of cells at the stump of the wound are typically referred to as a blastema. Basically, the blastema continues to grow and regenerate the limb in a way similar to fetal growth. Though mammals (including humans) also have some regenerative capabilities, they are much more limited. In addition, many regenerated limbs will also be mildly deformed (square fingernails). In axolotl salamanders, it was found that certain retrotransposons, such as the LINE-1 element, were prevented from "jumping" around in the developing blastema during limb regeneration. The two active proteins PL1 and PL2 were responsible for inhibiting the activity of LINE-1 and other such retrotransposons. Active retrotransposons can potentially rearrange the genes in germ cells can be detrimental to the mechanisms that allow the blastema to replicate and differentiate, so the PL2 and PL1 proteins play a vital role in their regenerative capabilities. When the PL2 and PL1 proteins were "switched off" in certain axolotl salamanders, their regeneration was much less efficient. Since mammals lack the PL1 and PL2 proteins and have a different LINE-1 retrotransposon, they have reduced regenerative capabilities.

Sources:

Wei Zhu, Dwight Kuo, Jason Nathanson, Akira Satoh, Gerald M. Pao, Gene W. Yeo, Susan V. Bryant, S. Randal Voss, David M. Gardiner, Tony Hunter. Retrotransposon long interspersed nucleotide element-1 (LINE-1) is activated during salamander limb regeneration. Development, Growth &amp; Differentiation, 2012; 54 (7)

Wei Zhu, Gerald M. Pao, Akira Satoh, Gillian Cummings, James R. Monaghan, Timothy T. Harkins, Susan V. Bryant, S. Randal Voss, David M. Gardiner, Tony Hunter. Activation of germline-specific genes is required for limb regeneration in the Mexican axolotl. Developmental Biology, 2012; 370 (1)

Odelberg SJ (August 2004). "Unraveling the Molecular Basis for Regenerative Cellular Plasticity". PLoS Biology 2 (8)