Chateau Tour Prignac, Medoc 2010, Bordeaux €12.99 (was €19.99)
Grape is the first answer you would probably give. Or perhaps the yeast, the fermentation method, or the winemaker’s philosophy.
However, if a wine has had contact with oak (new or old) this will have a much greater influence than you might imagine. For example, fine Rioja or Bordeaux would simply not smell or taste like they do without careful use of oak.