Aside from urban wildlife such as mice, opossums and raccoons, it is an unfortunate truth that vandalism and theft do occur. Consequently, you need to consider who has access to the garden site if it is not in a totally secure location. Chances are, if you have access to a rooftop, the neighbors will also. One would hope that they can trust their neighbors, but a garden located at street level is much more likely to be exposed to nefarious characters. This is especially true if the garden is part of a community garden project or simply located in public space.
Tenure is often an issue considered only when it is too late.
It would be very unfortunate to establish a garden in a location only to be informed that a large air conditioner, Dumpster, parkinglot, or statue of the landlord is going to be erected. Make sure that the location has some degree of permanency if you expect continuity in the garden. Similarly, if the garden is in a public space, such as the fire escape or pathway, be sure to meet all fire codes regarding obstructing exits and such. And when you are considering such regulations, it is extremely important that you know the live load of the support. In other words, how many pounds per square foot can the surface support.