There really is no simple, or even concrete, answer to your question. Using the standard definition, a bioregion is an area with a marked ecological and cultural unity. There are a myriad of features that can be used to define a bioregion.
Some people use watersheds to delineate bioregions, some people use landscape features.
Despite the relatively recent emergence of bioregionalism, these questions are not new. Robert Bailey's delineation of ecoregions in the USA using climate as a primary factor has become a widely referenced example, largely because of the systematic approach used in delineating these areas. However, if you change the relative importance of the factors which are used to delineate these ecoregions, you get very different boundaries. Scale is another factor that needs to be considered. A nested hierarchy is used in most ecoregional delineations.