Can anyone give me a good explanation of why HF is a weak acid?  It would seem to me that this should be an extremely polar bond, almost ionic in nature.  So why doesn't this acid completely dissociate in water?

While fluorine is the most electronegative element, you also have to
consider the fact that it hydrogen bonds, where other halide acids do
not.  It can hydrogen bond to itself, as well as to water.  Since the
Hs in HF are tied up in hydrogen bonding, H+ is not free to move about
the solution to affect pH.The hydrogen bonding also stabilizes the H-F bond significantly. So
despite the fact that the bond, on its own is the most polar, the
energy required to dissociate is much higher than in HCl or HBr.