I did a NSLI-Y program this summer (Journey to Jordan). Some things to keep in mind:
* NSLI-Y doesn't actually run any of the programs; it's the umbrella funding source for several programs, which are organized by non-governmental entities (like AFS, or in my case UW-Green Bay). However, you don't have to apply to the programs individually; you apply for an NSLI-Y scholarship and, if accepted, you will be sorted into one of the programs based on your language preference.
* No one really knows which programs are easier to get into, because there aren't statistics on how many people apply for each languages. However, Arabic and Chinese are the programs with the largest availability (I believe each language takes around 50 or more kids).
* On what the application committee actually looks for: MATURITY. I would say this is the biggest factor in the admission process, and it's impossible to get away with faking it, so I'll just say this: being a mature, passionate candidate will help you a lot more than previous language experience.
* On the program: All of the programs are different, obviously, but mine was a life-changing experience that I will never forget. I strongly encourage anyone with even a passing interest in foreign languages to apply for this program.