These can include:

Damage to the vessels to the brain in the case of stroke.

The bleeding and hemorrhage in the retina of the eye as well as glaucoma each causing blindness.

Dental problems such as chronic gum disease and periodontal infection may be responsible for instability in control of blood sugars and also lead to bacteria and plaque building up in the vessels of the heart.

Blockages in the arteries of the heart resulting in angina (chest pains), heart attacks and stroke.

Damage to the kidneys resulting in kidney failure.

Loss of sensation or chronic pain in the legs and feet called neuropathy. Poor circulation can also result in ulcers which are slow to heal and added to the loss of sensation, the threat of amputation from infections which are not detected or due to lack of circulation simply never heal,

Additionally neuropathy can even numb the digestive tract so the body is unaware that it has food ready for digesting.

Diabetics (as a rule of thumb) usually experience the beginning of some of these complications after about fifteen years, though many are able to delay them for much longer.

Studies have shown that stable or near normal blood sugar levels may delay complications, though there is no guarantee that a person, no matter how well they take care of thier diabetes, will never experience them. It may all come down to nuances in an individual's metabolism, heredity, or factors on a molecular level in the individual cells when absorbing food, insulin, or effects caused by swings in blood sugar levels.