I get asked this question a lot. With the advent of the critical system of wine, judging on a 100 point scale, it brings to mind the next question- "What differentiates an 80 point wine from a 90 pointer, from a 100 point one?" Good question. Here's my version of the answer- it's multifold.
A wine must have balance in order to be great. Simply put, no single aspect of it can be too far out of whack. It must have fruit characteristics, but not only those- there should be some semblance of secondary flavors, be them earthy, spice, or mineral. White wines must have some acid, but not so much it's stripping the enamel off your teeth. Red wines must have some tannin, but not too over the top. The temperature must be right- probably a bit cooler than you normally get on your reds, and a bit warmer than you normally experience on whites. Finally, the alcohol has to be in check. You should be able to sense it, but not have it burn the back of your throat.
This is harder to define. A wine should have something about it that makes the hair on the back of your neck stand up just a bit, in order to be great. I have always referred to this as "lift", and it usually is tied to the acid/fruit balance. Some scientists have attributed this to barometric pressure, which is just a wee-bit hard to control.
Getting away from the critical acclaim a wine draws, the situation that it is drunk under has a lot to do with the greatness of a wine. If it matches the food perfectly, and you are with loved ones laughing and enjoying yourselves, a wine tastes much better than if you are blind tasting it in a stark white room, everyone being quiet. I get this all the time- people who go on their honeymoon, and drink some everyday glass of wine looking over a gorgeous vineyard at sunset, and think it is the best damn thing they have ever tried. Those same people come home, buy the same bottle of wine, and try it when they are preparing their taxes, and wonder why it doesn't taste the same. The smell, sights, noises, everything of the experience plays into how you enjoy a wine. Also, don't discount food and wine pairings. Try as you may, a wonderful, bold, silky Barolo is going to taste bitter when you drink it with asparagus, or with something really sweet.
Je Nais se Quoi
There's a "certain something" that some wines have, and others don't. I don't know what it is, but when you experience it, you will understand what I'm talking about. It's the thing that you experience when you try a beautiful, handmade, cared-for wine that separates it from the junk that is being pumped out a million cases at a time. Notice that I did not mention price anywhere in this list. Price is irrelevant to the greatness of a wine.
The cool thing about wine is that it is always changing. Just when you think you have maxed out on an experience, one blindsides you that you think "wow, this could be it...the best". Or, you try a 100 point wine and think to yourself "really good, but not perfect". That is the thrill of the hunt, and why I love this industry.