I was asked recently at a tasting held by the Wine Club at London Business School. It's a straight forward enough question - just straight forward to ask and a little trickier to answer, I replied.
The key is to think of each bottle like a little art, history and science project.. all wrapped into one. History, as it often tells the story of a particular vintage (the year it was made), science (the vinification, the chemical balance of the grapes) and lastly, Art.
If you look at the 'greats' in each discipline, you can describe examples of why they are beautiful - a walk around Florence is enough for most - but critically dissecting 'why' is a little tougher. Most people agree the Sistene Chapel is a great work of art, but some modern art installations leave some cold and a little befuddled.
Wine is pretty similar. When that holy trinity of characteristics align and you have a great year, a great winemaker and a great climate/terroir, people can often refer to wine as a piece of art. Were someone to offer you a 1961 Chateau Petrus, most in the room would probably be in awe. Where it gets more fun, is the £7 viognier you just bought to experiment with from the local wine merchant.
So the way I answer the question is this : (1) Objectively - is the wine well made and (2) Subjectively do I like it. For the first point, you are looking for balance above all else. Not too heavy, not too light. Not too much alcohol, not too flabby in the glass. Not a punch in the nose with aromas, but more than just a hint of perfume. Or as I refer to it, the Goldilocks condition... it should be... just right.
Above all else remember the key message - it's your palate, it's your taste, it's your Tipple. If you love a wine then enjoy it.