The implications were - and are - stunning, and completely lost on many hierarchical pseudo-Protestant sects. Who decides what counts as a tithe? Who determines whether it's okay to watch TV after church? Who decides on such weighty matters as fish on Friday, or matzo in the Eucharist? Ultimately it devolves down to the believer (or more properly the specific gathered community to which he or she belongs). There is no elite class, no pope or apostle, to which all such matters must be referred to for an iron-clad ruling.
This was intended as a body blow to the Roman church. But I hadn't thought any further about Brother Martin's intentions beyond this. And in the raw it's an excellent principle, especially when applied to megalomaniacal Bible bashers. Think Flurry and Meredith (or Houston and Tamaki for that matter).
But alas, such far-sighted idealism was far from Luther's intent. Recently Bob Price pointed out that the doctrine of the priesthood of all believers was intended to empower the German princes who were backing the Reformation. Most certainly not the garden-variety believers, and not primarily the pastors and teachers, let alone the great unwashed peasantry ("murderous thieving hordes" said Luther) who were merely trusted to memorise the pabulum of the Small Catechism. No, it was all about political realities. A dire situation required that the Electors and princes of the Empire 'man-up' and tear free of Rome. For the nobility this would have been a frightening prospect, an extreme risk to their souls and an usurping of rightful religious authority. Luther's message was basically, "no worries!" Anything the purple-clad princelings of the church could do could equally be done with good conscience by the German nobility who constituted the 'left hand of God'.
What the Great Reformer didn't realise was that the concept would grow legs and be applied beyond it's intended audience, feeding out into the general process of democratisation. The petty tyrants of the European principalities have long since turned to dust, Gott sei dank. It was a prospect that sent Luther into conniptions.
The implications continue to echo down the centuries, as sweet as any rendition of Ein feste Burg.