I think story should ALWAYS be upheld as the most important element of a tabletop. Simply put, it's why the players (and the GM) are there to begin with, to play an awesome story. Now, as far as the importance of mechanics, I think it depends, not only on the players, but on what type of system you are using. If you're playing D&D (particularly D&D4e) then the mechanics are important, because they're there for you to follow, and, in my experience, typically make things easier to follow, especially when deciding the outcome of events. However, when you get other, more open systems like GURPS or Cortex, the mechanics can be played with a little more openly, and rules can handle being bent, and it will usually come out better, as long as it makes sense in the story.
Either way, i think the most important thing is that everyone at the table has fun, whatever that means about the rules and story.
