With mitral regurgitation or stenosis, blood backs up into the left atrium, the lungs, and the body (heart failure). The purpose of a normal mitral valve with normal function is to keep blood moving through the heart away from the lungs and to the body. You can be completely asymptomatic (no symptoms) and still require surgery in some cases to prevent long-term damage to the heart (if your valve is easily repairable and surgery is done at a high-volume mitral repair center). The symptoms, when they do occur, can be quite subtle because they usually develop slowly over time (even years). There are some conditions where symptoms occur suddenly (such as after a heart attack or rupture of the the parachute chords holding the valve in the proper place). Many of the symptoms below can also be associated with other conditions.
The important concept to understand is that you may have mitral disease and no symptoms or you can have very subtle symptoms that may not be apparent to you because they often develop very slowly. Unfortunately, mitral valve regurgitation can cause damage to your heart and body over time so recently there is an increased emphasis on early surgery to avoid these complications in low risk patients (low risk for surgery).
Shortness of breath (most common), especially with exercise or when laying down
Fatigue, exhaustion(less able to walk as far or exercise as much)
Palpitations (due to an irregular heart beat)
Cough, often at night
Swelling of the feet, ankles, and legs
Excessive urination, especially at night
If you have any of these, you really should see a physician or other health care professional as soon as possible so he/she can figure out what the problem is. There are many other conditions that can cause these symptoms other than mitral valve disease.