Is it legal for me to stand on public property near a doctor's office and express my negative feelings?

Picketing, which is what you want to do, is legal in the U.S. as long as you do not interfere with or get in the way of business, and as long as you are not starting a riot or anything. You can say what you feel, as long as its truthful. a man in our town picketed a car dealership a few years ago, for selling him a lemon. He parked the car on the street near the place with a huge sign stating how he was sold a lemon, and people should not do business there. After a few days of this the dealer finally gave in and gave him a new car, because it was effecting their sales so much.