These are the inherent features of the system, and thus, potentially, the strongest ground on which to argue the resolution.
1. Detention in juvenile facilities while awaiting trial.
2. Media blackout. (Juvenile defendants' names are not generally made public.)
3. No jury trial.
4. Separated from adults when incarcerated.
5. Possibility of having criminal record expunged upon reaching the age of majority.
6. At present, defendants charged as juveniles cannot receive life without parole or the death penalty.
It is important to note that the Negative does not have to uphold the status quo. There may be other potential differences, from a Negative perspective, that are not currently features of the U.S. juvenile justice system, but should be. For instance, the Neg could argue that juveniles should not receive any due process rights, or that juveniles should be punished more harshly than adults, and still negate the resolution. (Such a "turn case" could be risky, but might catch some Affs napping.)
These are research-dependent, and thus contestable differences.
1. In interrogation, juveniles may be more likely to waive due process rights due to their relative ignorance of the proceedings, perhaps combined with manipulation or coercion by adult authorities (administrators, police, prosecutors).
2. In the current system, juveniles may have a greater likelihood of informal, ad hoc solutions. According to the Oxford Companion to American Law,
Following the arrest of a juvenile, law enforcement officers may either send the case to juvenile court or divert the case out of the system. Although most cases are routed into the courts, substantial numbers of youths are released following their arrest. For cases directed to the juvenile courts, an initial decision is made about whether to proceed to a formal hearing, dismiss the case, or handle the matter informally, perhaps through referral to a social service agency. A large number of cases are dealt with informally, often ending in dismissal or an agreement between the juvenile and the court. A formal procedure involves either a waiver hearing to determine whether the juvenile should be ordered to stand trial in adult criminal court or an adjudicatory hearing before the juvenile court judge.3. Juvenile punishments are considered by many to be more lenient--or even too lenient.
4. Juveniles might not be able to handle incarceration with adults, for psychological or developmental reasons, perhaps making it a form of cruel and unusual punishment.
5. If incarcerated with adults, juveniles would likely face the prospect of abuse by other prisoners or guards.
6. If incarcerated with adults, juveniles might be more likely to be granted parole.
7. Perhaps most important from a utilitarian perspective, juveniles incarcerated with adults may have higher recidivism rates.
Regarding the empirical differences, I haven't found all the research out there to warrant these intuitions (although #7, I believe, is pretty easy to warrant). If you find any, feel free to post it. Of course, I'll have more specific, detailed information in the coming days and weeks.