- Protection against the effects of "congestion," which in practice means automobile traffic and parking spillover. This partially accounts for opposition to higher-density projects which are clearly indicative of increasing property values: for example, new luxury apartment towers in Queens' Long Island City neighborhood, where existing residents demanded large quantities of garage parking even in the absence of city parking minimums. Traffic associated with "apartment houses" also made in appearance in the Euclid v. Ambler decision as a justification for density limitations.
- Exclusion of the poor, for both quality of life and economic reasons. The latter point relates to property taxes: even if upzoning increases land values, it may be that net property tax revenue, per resident, will fall, resulting in a decline in the quality of city services and the necessity of a tax hike. Exclusionary zoning could then be seen as a way to keep property taxes low -- at least for so long as the mounting externalized costs of a very low-density development pattern can be postponed.
- The "neighborhood character" argument, which Benjamin Hemric alludes to and which is captured in this 1994 paper and in the old Disney animated film Benjamin mentions (The Little House). This I think gets at the heart of what zoning means when it states an intention to "conserve" the neighborhood. In essence, the neighborhood as it exists meets the density preference of the existing residents, and that any further densification would result in a perceived decline in quality of life, regardless of any increase in property values. Cashing out by selling and moving away is not always a desirable solution if one has a long-term investment in unique neighborhood institutions and amenities. To some extent this is the same argument advanced by the foes of gentrification.
- Finally, protection from harmful or noxious uses. Although this reason is far less significant than it was 100 years ago, when promoted as a pre-emptive measure against a select group of activities it has a certain logic as an efficient alternative to costly and drawn-out nuisance lawsuits.
In answer to the last of those, Benjamin's suggestion of reviving a variant of the original, 35-page 1916 New York zoning resolution is an interesting one. He's not the only one to have this thought: a 1993 City Journal article on New York's modern zoning code, still very relevant today, had positive things to say about its simplicity, predictability and clarity. I do have some thoughts of my own on all this that I'll get around to in a post or two.