What chemical similarity is shared by all acids?

Acids share a number of similar chemical properties.
Some of these properties are:
1- All acidic solutions have a pH value less than 7
2- Acids react with metal carbonates producing a salt, carbon dioxide and water
3- Acids react with bases producing salt and water (This reaction is known as neutralization reaction)
4- Acids react with reactive metals producing salt and hydrogen gas

They are all full and don't need any more electrons in there outer shell.