Bad spark plugs can cause a number of instantly obvious problems. If a spark plug isn’t completing burning the air fuel mixture, you will see a marked decline in horsepower, torque (acceleration), and fuel efficiency (gas mileage). Unburned fuel can form deposits on exhaust valves that will interfere with exhaust gas flow, and also the ability of the valve to completely close and seal. Over time, the excess fuel in the exhaust stream can begin to foul the oxygen sensor. Most problems that a bad spark plug can cause are repaired relatively easily. However, a spark plug that is left in the engine for an extended period of time can cause problems with piston rings and rod and crank bearings. These types of problems are more serious, but still not permanent.
For more info, see What is Involved in a Car Tune Up