I agree with the above. Of course strong grades, LORs, test scores, etc. won't hurt...but there is no one thing that will get you in or keep you out. A long list of publications is not a prereq, but some research experience is useful because it helps to solidify your case for wanting to participate in the program. If that part of your application is lacking, they may wonder why you are willing to take a year off of school in order to pursue something you may end up hating. If you don't have any experience, you need to do some convincing as to why you think this is an important part of your training. Most of all, they want to see your committment and ability to think on your own.