First of all, here's the bad news for wine plebs like myself: All of those "flavor notes" that people talk about really exist, even at the chemical level. Chocolate! Tobacco! Grass! What?
Some scientists estimate that a single glass of wine contains thousands of different chemical compounds. Those chemicals are determined by the soil the wine grape is grown in -- which can contain a host of unique minerals to influence the fruit -- the grape itself, the climate, and the fermentation process, where crushed grapes provide sugary fuel for yeast, which in turn produces alcohol. The barrels and environment a wine is aged in can contribute new chemical compounds as well.
And most wines probably have mostly the same chemicals inside of them -- it's the concentration of one compound over another that makes the biggest difference.
One compound, 3-isobutyl-2-methoxypyrazine, tastes like bell peppers and is unpleasant at high concentrations. Experts have figured out that grapes grown in the sun (as opposed to the shade) are less likely to have too much of it.
Then there are tannins. Even if you've never sipped a red wine, you probably know the taste of tannins from that bitter edge you get from strongly brewed tea. They can even make you feel like your mouth is dryer than it was before you had a sip. That's because tannins bind to the proteins in your saliva -- probably an evolutionary tactic developed by grapes to keep animals from wanting to eat them. Winemakers like tannins because they give the wine a longer aftertaste, but too high of a concentration will leave you pursing your lips.
Tannins are often touted as providing many of wine's potential health benefits -- some people even take baths in wine in the hopes of soaking up some of the chemical compounds therein -- but the scientific jury is still out on just how good wine might be for us. While we wait for more data, we're best off drinking in moderation.