I have to agree that RN doesn't mean a whole heck of a lot until you get more than a few year of strong experience--preferably in acute and/or critical care--or some area that is a bit more clinically and intellectually and accountability challenging, so to speak. I think those RNs/BSNs or ~ that have a better chance of MS entrance--besides the obvious higher priority req's (cGPA, sGPA, high MCAT, etc), are those that found out a matter of years after becoming and working as RNs, that the were/are drawn to something more medically/scientifically hands on--more in terms of medical/scientific application, so to speak. So, they then are willing to go, at this point, through the process of being beaten up to become a physician. And they have witnessed what so many MSs, residents, and fellows have gone through--that, is, to a greater degree than say the average applicant.
Something hits you as you practice in nursing. It's usually during practice in a more intense area that you realize you think a bit differently from your fellow RNs. It's not a superiority thing--it's just how you prioritize things in your head and thinking processes re: the pts. There is nothing wrong with being a RN. Many aspects of being a RN, particularly in certain areas--at least for me--were great and even exhilarating at times. (And the people/family-dealing aspects of care will always be there, for the most part; whether you stay a RN or become a physician.) But something resonates in you to a degree and intensity you had not anticipated--and you may not have initially realized--your focus in somewhat different. It can be a little tough to explain; but you don't know this until you function as a RN, preferably in certain settings.
If you complete the BSN, w/o first practicing for a decent period of time as an RN, and then you jump to med pre-reqs, sure. You could still get in. . .depending on various factors. But there are those that will look suspect at your degree choice--looking at it as some lame attempt at a stepping stone. BSN is not a stepping stone to medicine. No way, no how. It's a different focus. People are entitled to major in what they will; but it may look like you made the commitment to nursing w/o the commensurate dedication to the field.
Of course, that's only one perspective. I am certain others have gotten in to MS with an undergrad in nursing and limited experience. But it's better to get the experience after passing your boards and working for a while. All I am saying.