What exactly made pre-Euclid development in American towns and cities more aesthetically rich than so many of the sterile developments of today? I think one of the most significant factors was the inherent balance between uniformity and individuality that characterized the legal framework of traditional town planning: the paradox of organic urbanism.
Prior to use zoning, certain legal requirements enforced a broad uniformity among neighboring buildings that created aesthetic harmony: lot sizes tended to be uniform as subdivisions of paper streets were filled in; fire codes limited building heights to the reaches of available equipment; air and light requirements limited density. Presumably, the codified requirements of the 19th and early 20th centuries evolved from the common law traditions and practical constraints of town-building that had controlled development in earlier times. But while these rules defined the corners of the working canvas, the work, so circumscribed, was as varied as those who built it. Uses were unregulated, except to exclude nuisances; architectural styles were driven by the tastes of individual builders and buyers; attachment, detachment, and maximization of coverage were driven largely by market conditions.
Other factors contribute to the abiding appeal of older neighborhoods: the availability of natural materials like clapboard, limestone, and slate, at the times of their construction; the rich detailing of a design that was intended for pedestrians rather than passengers in speeding cars; private law controls, within subdivisions, that ran with the land; generally, the relics of another time. Variety also accrues over time, as identical buildings are modified in different ways. But I suspect the most significant factor in the richness and authenticity of older towns and city neighborhoods lies in the inherent balance of the legal framework that shaped them: one that regularized the many canvases of a neighborhood, then stepped back and allowed them to each be developed according to a wide variety of individual tastes and specific economic demands.
Today, many urban redevelopment projects seek to recover the richness and variety of traditional neighborhoods, but their planners often fail because they attempt to exercise complete control over an entire, unitary project. Often, it might be the case that defining the scale of development, the quality of building materials, and a set of specific exclusions would be enough. Beyond these basics, planners could exercise a light hand, and let the marketplace of individual styles and economic necessities fill in the details, resulting in a variety that more thoroughly expresses the character of the community.