Brewers call malt "the soul of beer" but they might also add that malt contributes mightily to the different personalities we expect from beer. Of all the barley grown, only one-quarter or less is used for malting. The rest is used to feed animals. Barley is well suited for malting because it has the right components for yeast nutrition, it tastes good, and it has a solid husk (protecting it at harvest, then later aiding the brewing process). Barley is first of all divided by how many rows of grain there are in each ear -- either six or two.
Two-row is plumper and responsible for a softer, sweeter flavor. It is regarded as higher quality and long has been the standard in the traditional brewing nations.
Europeans brewers are not alone in calling six-row barley less refined, and a beer made only with six-row is more likely to taste grainy and will probably show chill-haze because of excess proteins. In moderation, it lends a firmness and husky character to beer, which some ale brewers prefer.