When I write a simple function in Scheme and find myself wondering whether I might later want to swap in a different algorithm at runtime, I don't start making a pluggable architecture right then. Because I don't need to. The function I wrote can simply drop into whatever pluggable architecture I write later.

In Java, if I think I might need a pluggable architecture later, I'm tempted to go ahead and make it from the start, since my object model will need to support it.