Be careful with edges, such as manholes and the junction between pavement and a bridge. If the edge is tall enough and you hit it with too much speed, you will compress the tire/tube against the wheel and get what is known as a pinch flat or snakebite flat. There will be two small holes on the inner circumference of the inner tube.
Don't run over plastic shopping bags, a/k/a Arkansas Tumble Weeds. They can get wrapped up in your chain/rear derailleur and bring you to a quick halt.
Watch out for sticks/limbs as they can get hung in your chain, spokes, and derailleurs.
Gravel on otherwise paved roads can be deleterious to control of your bike, especially in corners. Larger rocks can cause pinch flats.
As you get more miles in, you'll learn more about obstacles, what to avoid, and how to react.