Firstly, Ionic solids can't conduct electricity, this is because of the close packed structure partly caused by the strong attraction between the delocalised electrons and the positively charged ions in the lattice structure (the close packed structure is also partially due to the fact it is in a solid state, it's not just solely to the ionic bonds). As the structure is so close packed, the ions are not free to move so electricity cannot be conducted. Secondly, high melting and boiling points (counts as a physical property as it can be observed) as there is high attraction between the oppositely charged entities in the ionic lattice, added to by the sheer number of ionic bonds and resulting electrostatic attraction in a lattice (aka strong and plentiful ionic bonds) mean lots of energy is needed to overcome these bonds in order to change state. Thirdly, they are usually hard and brittle as the lattice is a huge structure that is held rigidly in place by the strong electrostatic attraction arising from the ionic bonds.
