Object keys don't need to be quoted if they're valid identifiers. Yes, even reserved keywords are valid unquoted keys in ES5 [§11.1.5, §7.6].
[TODO: Unicode characters and escape sequences aren't yet supported in this implementation.]
Strings can be single-quoted.
Strings can be split across multiple lines; just prefix each newline with a backslash. [ES5 §7.8.4]
Objects and arrays can have trailing commas.
Both inline (single-line) and block (multi-line) comments are allowed.
Numbers can be hexadecimal (base 16), and they can also begin with a leading decimal (e.g..5).