Usually when somebody refers to a "heritage" station they're talking about an old tired AM that nobody listens to anymore that just got sold to a Jesus outfit or had its power reduced (like WOWO) to benefit another station. I can't think of any time when I've seen the word used in a context that wasn't negative.
I think the Wikipedia entry is bogus, or at least the FM part: "In commercial radio, heritage station is the term for an AM radio station that has broadcast a specific format for a sustained period of time, since its beginning; usually since the 1930s or 1940s; in some cases the 1920s. Also, the call letters will have remained the same for most of the station's life. The term is also used for FM radio stations that have broadcast a specific format, usually since the 1970s or 1980s."
I think the Wikipedia entry is bogus, or at least the FM part: "In commercial radio, heritage station is the term for an AM radio station that has broadcast a specific format for a sustained period of time, since its beginning; usually since the 1930s or 1940s; in some cases the 1920s. Also, the call letters will have remained the same for most of the station's life. The term is also used for FM radio stations that have broadcast a specific format, usually since the 1970s or 1980s."