Actually, the specific grapes (and where they are grown) define the product more than anything. Next would be the process, and next would be the particular blend that the palette of the winemaker (often following a house style) strives for.

The fact is, wine of a certain character is made from the Champagne region in France. Others can copy the process, but the grapes, even if the same exact varieties, are not grown in the soil of the Champagne region, in the climate of the Champagne region, it won't be exactly the same stuff, and it will taste different, even if you do not have the palette to discern that difference.

Now it's true, that there are similarities. But Domaine Chandon Brut, made in Napa, from the same grape varieties, as Moet Chandon Brut, made in Champagne, under the guise of the same parent company, tastes different. Personally, I actually prefer it, because the grapes are invariably riper and fruitier than the grapes grown in Champagne, and the Champagne product is more austere and yeastier, because there is less grape flavor to be overcome, but that's my preference - they are NOT the identical product.

The same is true of other French Champagne houses that also make similar wines in places like California, Spain, Argentina, and Chile under their own subsidiary companies.

The fact is, the French have been somewhat more successful in protecting the name Champagne, than they have with other famous place names, such as Chablis and Burgundy and Sauternes. But I can assure you, Chablis from the small town of Chablis, does NOT taste like "Chablis" made anywhere else on the planet either.