Informally, the preferred syntax of a language consist of those features which can be accessed by the programmer with a minimal amount of typing (compared to other features of the langage). Other features may have non-preferred syntax (though be implemented by the language itself); still others may require an external implementation outside the language proper or its libraries.
Important, because SyntaxMatters. One can often deduce what paradigm a language belongs to by examining its PreferredSyntax.
(It should be noted that we are discussing concrete syntax here, not abstract syntax).