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The extra red blood cells make your blood thicker than normal. As a result, blood clots can form more easily. These clots can block blood flow through your arteries and veins, which can cause a heart attack or stroke. Thicker blood also doesn't flow as quickly to your body as normal blood.
A clot that breaks away and travels to the heart or lungs can cause a blockage which can lead to death. The other type of thick blood would be blood that contains too many red cells. It really is thick and the only treatment is to the bleed the patient periodically. The patient will have a unit (pint) of blood drawn every week or so until his blood count is down to normal. The patient should be under the care of a doctor who specializes in hematology (study of blood).