Wine production in the New World has been going on for well over one hundred years, and we are still being compared to the Old World. Tradition, and tried and true methods, are difficult to undermine, yet New World wines constantly 'go rogue' and attempt what "they" say shouldn't be done. A wise old man once said you cannot color outside the lines without knowing the lines in the first place...and that is just what Old World winemakers symbolize - the lines.
The Old World we refer to in winespeak is the regions of Europe and parts of the Near East, Northern Africa and the Mediterranean. It is here that wine has been made for centuries, and here where many traditions have been cemented. While the laws put in place to regulate the industry vary from country to country, they stem from a common thread: to maintain the integrity of place, a concept they call terroir.
Many Old World wine laws put restrictions on what type of grapes are grown in a region, how much can be planted, when to harvest, and even winemaking techniques. Within these standards are stricter rules that determine a wines "quality" level. For example, in France, a winemaker strives to reach the highest level - the AOC (appellation d'origine controlee), a label that connects the terroir of a region with quality wine. In theory, a consumer will see a see bottle of Chateau Lafite-Rothschild and know it is a premier cru from Bordeaux, a fine French wine, because it is an AOC labeled wine.
This shows one of the two main differences in Old World versus New World wines: labeling by region alone as opposed to region AND varietal. The Old World wine labeled as a Bordeaux tells the consumer it can be made only from a distinct and specific group of grapes. Since we place no laws against what and where vineyards can be planted in the New World, we find ourselves with the options to buy cabernet sauvignon from places as diverse as Napa Valley(United States) to Maipo Valley(Chile) to Barossa Valley(Australia).
The second main difference, of course, is taste. With stringent rules in place to govern all aspects of the wine, from vineyard to bottle to labeling, in the Old World, it's only natural that the flavors of the wine will differ. As a rule, Old World wines have a tendency to be "earthier" (as opposed to fruity) and more complex, with higher tannins that lead to a longer aging time. As with all wines, it really comes down to taste and personal preference.
Have some fun of your own with the Old World/New World debate. Visit your local wine merchant and get a bottle of each of your preferred varietal (cab sauv, pinot gris, chardonnay, whatever you fancy!) and have your own little Old World/New World showdown. Let me know how it goes!