One way of looking at what's distinctive of Platonist/Rationalist/Realist views, which can be collectively referred to as "the bad views," is that they prioritize identity over similarity, when similarity is the more fundamental concept. The Platonist characterizes similarity as identity in some respect; in the view advocated in Plato's writings, they are similar because they participate in identical forms. All forms of rationalism have some form-like elements which play the same role, and even modern, supposedly scientific varieties of realism get all superstitious about "natural kinds," basically forms in modern dress.
Similarity, however, is the more fundamental idea. We only arrive at the notion of things sharing the same property on the basis of observing their similarity. Plato himself saw this, which is why he made the unsatisfying move of locating the forms outside of the world of our experience, because it is so obvious that we do not experience them in this world. However, while the various mystics have proposed a number of different unsatisfying ways we might perceive the genuine, unchanging forms, the only ones who have claimed that similarity is something truly universal, objective, and unchanging have been those who interpreted it in terms of presence of identical universal, objective, and unchanging forms. Otherwise, it is obvious that our recognition of similarities is heavily influenced by our contingent interests and our past experiences. Thus, the similarity view encourages pragmatism, empiricism, and anti-realism, "the sensible views," to give them an all-encompassing name.
I know David Armstrong makes much of the need for identical properties to ground similarities between things. I'm trying to think of who else has seen the status of similarity vs. identity of properties as a crucial perspective on the great philosophical divide.