A macguffin is not "anything" that drives the plot along, it's an "unknown" thing that drives the plot along. In North By Northwest for example, the macguffin is the mysterious "microfilm" – we don't know what's on it, nor do we need to know, we just know that it's hugely important. The Pulp Fiction briefcase is another in a long line of these mysterious elements. Part of what makes a macguffin so interesting is its lack of definition – people are risking everything, killing, dying for this thing that remains (at least for the audience) totally ambiguous.