Why does mercury have low melting and boiling points?
Mercury is different!  It is not as reactive as its neighbors in the Periodic Table, it doesn't conduct heat and electricity as well as other metals, and it is a liquid unlike other metals.  The electronic structure of mercury is $\ce{[Xe] 4f^{14} 5d^{10} 6s^2}$, so the first thing we notice is that all of its orbitals are full, there are no unpaired electrons in mercury.  This helps explain mercury's reluctance to react with other materials or form bonds with other mercury atoms.  Most metal atoms share their outer electrons with other metal atoms, in fact, all of these "shared" metal electrons exist as a diffuse "sea" of electrons.  It is the extended nature of this sharing that makes metals good conductors of heat and electricity.  The fact that the metal electrons are shared provides a strong bonding interaction between metal atoms that gives metals their solid structure.  None of this applies to mercury.  With its filled shell electron configuration it is very reluctant to form bonds even with other mercury atoms. 

One additional, interesting effect makes mercury even more reluctant to share its electrons.  Special relativity suggests that for larger nuclei (and mercury with atomic number 80 is large enough for the effect to apply) electrons will begin to travel closer to the speed of light.  In turn, their mass will increase and as mass increases, orbital radius decreases.  The electron is closer to the nucleus, more attracted to the nucleus and less available for bonding.  Here is a nice reference that explains this relativistic effect and how it applies to mercury in plain English and a bit more detail.

These two effects, filled outer electron shell and contraction of the outer orbital closer to the nucleus, combine to make mercury reluctant to form bonds, even with other mercury atoms.  It is this lack of electron bonding between mercury atoms that makes it melt and boil at such low temperatures.