Unconformities occur when there are no rocks preserved from that time period, because no rocks formed or because all of the rocks from that period have eroded away. Little unconformities happen all over the place, pretty much all the time.
The Great Unconformity is different than these little unconformities for a few reasons: It is one of the longest known, almost everywhere in the globe exhibits it, and perhaps most importantly it divides the rocks with familiar fossils from those with no fossils or only fossilized bacteria.
