Without acid, coffee will fall flat on your palate and taste dull. The acid gives coffee a dry and bright flavor and usually is the first sensation on your palate when tasting. A little acid really is a good thing!
Coffees high in acidity are generally from Central America and East Africa. Some high acidic coffees that come to mind are from Guatemala, Costa Rica and Kenya. There seems to be a correlation between acidity and coffee grown at very high altitudes and in mineral rich volcanic soils. Coffee from Brazil, Sumatra and India generally have a naturally low acid content. They are from lower grown regions and when they are moderately roasted they will be very pleasant and flavorful. (As long as the beans are not burned this is usually a great cup of Joe)
The acid content of a coffee also depends on the roasting degree. A darker roast will generally have less acid. Dark roasting is the most efficient way to kill the acidity and also mask it.
There are many ways to reduce acid by adding chemicals and compounds. I personally am not interested in drinking coffee that is not natural.
In summary, for those of you that can tolerate a high acid content I applaud you because you can enjoy some awesome coffees from around the world and keep an open mind on their quality. I am intrigued by coffee acidity and the complexity it adds to the whole cupping experience.