It's pretty common to see people photographing with tripods in Central Park--large format, 35mm, big glass for bird photography. You even see pros and students doing model shoots, and during certain seasons, Chinese wedding photographers and videographers are ubiquitous. You're not too likely to get stopped in Central Park if you're not using lights.
If you're photographing a bridge in a very visible spot where there's a lot of traffic, then you're more likely to get stopped. I'm photographing the Riverside Drive viaduct all the time, and I've only been asked questions by passersby interested in the big camera. In certain places, like the West Village, you'll see painters with easels set up in the street.
In general you don't want to be set up with a tripod and an LF camera in many of the places you'll be asked to pack up anyway, just because there are too many people around, and it's easy for things to get jostled, and you have to watch everything more carefully.
In some places it's a commercial issue. Some places it's more a safety issue, and in some places it's a "Homeland Security" issue.