Numerous. There are many complications of diabetes. Most important, it is considered the same as if you had a heart attack. That means your doctor will treat you as if you already had a heart attack even if you didn't actually. Diabetes also affects the kidneys, eyes, nerves, and stomach all due to its effects on your blood vessels. All of these problems can be mitigated by controlling diabetes.
Eyes, Kidney, Stroke. Diabetes can hurt every tissue in your body. Number one cause of death from diabetes is myocardial infarction. The number one reason people go blind in the USA between ages of 25-65 is diabetes damage to eye. The number one reason people have kidney failure and dialysis is diabetes damage. The group of people that most often looses their limbs, people with diabetes.