Covalent compounds (a.k.a. "molecular compounds) consist of individual covalently-bonded molecules. The covalent bonds within the molecules are very strong. On the other hand, the forces between the molecules are very weak. It's the weakness of the attraction between molecules that give covalent compounds many of their properties.
Disclaimer: There are many exceptions to these rules, so if you see something that doesn't follow them, don't panic.
- Low melting points and boiling points: Covalent compounds melt and boil when the intermolecular forces between them are disrupted and the molecules can move away from each other. Because intermolecular forces are weaker than the electrostatic interactions in ionic compounds, covalent compounds have lower melting and boiling points.
- Low enthalpies of fusion and vaporization, usually one or two orders of magnitude smaller than they are for ionic compounds. Same reason as #1.
- Covalent compounds tend to be soft and relatively flexible.
The weak intermolecular forces makes the solid form of covalent molecular compounds easy to distort or break.
- Covalent compounds tend to be more flammable than ionic compounds. Carbon and hydrogen are needed for things to burn and they have nearly identical electronegativities - as a result, the organic compounds containing both are covalent and can burn.
- Poor electrical and thermal conductivity. Ions are needed to conduct electricity in an aqueous solution. Molecular compounds dissolve into molecules rather than dissociate into ions, so they usually do not conduct electricity very well when dissolved in water.
- Many covalent compounds don't dissolve well in water. There are many exceptions to this rule. Examples of molecular compounds that don't dissolve well in water are oil and polymerized plastic.