Water is the blood in beer.
The water across the country varies greatly and the water will have a direct affect on the taste of the beer. Hardness, which is composed of calcium and magnesium ions has to be considered. Many brewers like the water to contain at least 50 mg/l of Calcium, but too much can be detrimental to the flavors because it lowers the pH of the mash. Similarly, a little Magnesium is good, but too much can create a bitter taste. 10 to 25 mg/l of manganese is most desirable.
Sodium can also be a contaminant that can create a metallic taste, which is why smart brewers never use softened water. It's almost always a good idea to keep sodium levels below 50 mg/l. Additionally, Carbonate and Bicarbonate are desirable at certain levels and detrimental at higher levels. Darker beers with high acidity sometimes have up to 300 mg/l of carbonate, while IPA's may taste best at under 40 mg/l.