What makes a great par 3 is dependent on the other pars 3's, they must be looked at as a collection and must be distinctly different than each other.
O, and it must have all that other good stuff people have been talking about.
I agree with Ryan and Tim G., that many par 3 holes stand out when they're a part of a collection of.
However, I also think one of the other key features of a solid par 3 is its context in the bigger picture. For instance the short #13 at Merion-East and the short #8 at Olympic-Lake. In the midst of a match, I think players think they need and can make birdie here since they'll be holding a 8I, 9I, or PW in their hand, and sometimes that little extra "pressure" can force them to make a mistake, which on these two holes can definitely result in a bogie or worse. While they're not long and won't beat you up w/ brute force (like Merion's #17), they can subtly destroy you.