My reading of NU results threads (both RD and ED) for the last couple of years shows me the following. Now, I realize that this is a small sample size because only a fraction of applicants share their information here … But within this caveat, If you have the stats – probably 3.85+ unweighted GPA, 34+ ACT, good course rigor, and some extra-curriculars – you have a pretty good chance of getting in ED.
RD is a completely different story. You probably have to be filling a need in the class (adding diversity along some dimension) and appear highly interested in NU. It is very common for a high stat applicant to be rejected otherwise. Surely, NU cares about getting more minority students, more low income students, more from under-represented geographical areas such as the south, more women in engineering (like all schools), and so on. Yield protection is also important.