What do you look for in a sequence diagram to indicate that the system is well architected?

What good-practice-indicating idioms do you look for in a sequence of method invocations, as depicted on sequence diagrams?

Look for the telltale central controller.  A well-factored system will tend to have an even allocation of responsibilities among the classes.  A poorly factored system instead tends to have a sequence diagram with many messages going to or from a class in the middle, which is frantically getting messages, making decisions, calling underlings, and generally running everything.
Not coincidentally, that controller class also tends to be big, have lots of procedural code, and generally look like FORTRAN in a funny hat.