If the water temperature is too hot, you'll end up with an over-extracted cup. Over-extraction is when the compounds from the coffee grounds are released too quickly, which tend to make the cup bitter and overly sour. Keeping the water just under a boil will help make sure the flavors are extracted as evenly as possible, which is why the ideal coffee brewing temperature is 96 degrees Celcius. Letting the water boil for too long can also ruin the cup, as boiling tends to concentrate the mineral content of water.