I would have to agree that a serious cyclist is someone that rides regularly. A cyclist could own a cheap bicycle and use street clothes and still be a serious cyclist. Alternatively, a person could own a $4000 road bike and a full kit and not be a serious cyclist if they only ride once a month or so. Of course, there are many serious cyclists that own expensive gear.
I do have to admit, I view the commuters that ride all year around in any weather condition as being the most serious cyclists.