I am in partial agreement with Sporn. Though I do believe that thinking too much about gameplay can hinder the flow and limit the possibilites of your story, omitting the gameplay aspect until after the fact can be just as detrimental. I have played a few games where, though the storyline was fantastic, there were plenty of gameplay elements that could have been fixed, or altered (the first that comes to mind for me is Persona 3. Though a good game, sometimes it felt like there was more story telling going on than actually gameplay. As if they made the story, then stuck in gameplay ideas).
I, personally, believe that equitable amounts of love should be shown to both.
I don't want to say that thinking of the story first and gameplay second (or vice versa) is a bad thing. Good things can come of it, and for some people, that is simply how they think. I would be mindful of both.