About the lack of a “J street” in DC, the story I always heard — and one that’s too good to check — is that L’Enfant had some sort of tiff with John Jay, the Federalist Papers author and the first Chief Justice of SCOTUS. Thus when designing the city, he left J off the otherwise alphabetical map of streets in D.C. to snub the man.
UPDATE: Yup, too good to check. And a number of readers have noted that Jay was technically the first Chief Justice of the United States, not SCOTUS.