There are three main factors that influence the quality of a qood wine. These are: storage temperature, storage humidity and lack of sunlight (more precisely ultraviolet light). The optimal temperature for wine storage is about 55 Fahrenheit degrees (take or given 3-5 F deg), the optimal humidity is about 75% (take or give 5%) the sunlight need to be avoided at all costs. Outside these conditions the wine depreciates very fast. High temperatures ages the wine too fast, colder temperatures causes the loose of aroma, higher humidity destroys the storage barrels, lower humidity destroys the cork of the bottles, sunlight radiations destroys the molecular structure of the wine and its aroma.
All these optimum conditions are met especially in cellars that are constructed in the northen part of the buildings. The temperature and humidity are constant during all day and nigh and the sunlight is avoided.
Valentin68 is right on the money, except for one thing, this only applies to the Northern Hemisphere. If you're in Australia, New Zealand, or anywhere else in the Southern Hemisphere, you want to put the Wine Cellar on the South Side of the building.
However, Valentin68 is right, especially about temperature and sunlight. Significant temperature changes have a degrading effect on wine, and even basements with a Southern exposure (in the Northern Hemisphere) can be subject to significant temperature fluctuations due to the sunlight striking that side of the building, even if there isn't a window. Sunlight also has a deleterious effect on the wine, and even the slightest crack in the wall could let it in, and the South side of the building is exposed almost the entire time the sun is up, year round. The North side of the building almost always points away from the sun, so there's less likelihood of accidental sun exposure, and temperatures tend to remain constant.