What makes a successful downtown?

The first time I walked downtown Nashua NH end-to-end (from historic Railroad Square on the north to City Hall on the south) I felt at home. Add the availability of a new condo building on the Nashua river at the north end and a walkability score in the 90's and I was sold. I bought a condo, went back to Florida to pack and moved to New Hampshire.

I have driven all over the US and visited many large and small towns. Some have found the right formula to create a livable, walkable, fun, vital, safe downtown. But many others either have missed by a mile or don't even understand the concept. Some cities are sure they have it -- and don't; other cities think they don't have it, and do.

What makes the difference in having or not having a successful downtown? Is it history? an aggressive downtown revitalization program? money? dumb luck? I wonder if there is a common thread among the cities with successful downtowns....