Since the Quran doesn't state any specific punishment for not wearing the hijab..?

If Allah says to wear a hijab, then you obey. You shouldn't ask for what punishment there is. You only have to wear a hijab in the presence of non-muslims, other muslims, and your relatives. You don't have to wear it in the presence of mehram( for example people you can't marry: your dad, your brother, your grandfather, etc.):]