You've got to remember that a metal is basically a lattice of positive ions in a continuous sea of negative electrons that act like a kind of 'glue'. In an ionic or covalent compound, you have to actually break bonds in order to change the position of atoms, which requires a lot of energy. In a metal, since the bonds aren't directional (the positive ions are electrostatically attracted to a continuous sea of electrons) you don't have to actually break any bonds in order to move metal ions. The fact that pure metals have a very similar crystalline structure helps as layers can simply slide over eachother.