Balance
Balance is about how a wine's fundamental elements work together, rather than how they taste on their own. The most important building blocks are sweetness, acidity, alcohol, tannins (in red wine) and the fruit itself. A wine is in balance when none of these stands out and dominates, but instead they play together into a whole that feels complete and rounded in the mouth. Think of it as a well-adjusted set of accounts, where sweetness and fruit are kept in check by freshness from the acidity, and where the alcohol carries the whole thing without burning.
Balance is decisive for what you experience, because it determines whether a wine feels harmonious or strained. A wine with high acidity but little fruit can come across as sharp and thin, while a very ripe, alcohol-rich wine without acidity easily becomes heavy and dull. You can often sense imbalance more clearly than balance: a stinging warmth in the throat suggests too much alcohol, a biting sourness suggests a lack of fruit to rest on. The climate plays a part, since cooler areas typically give more acidity, while warmer ones give fuller fruit and higher alcohol.
A widespread misunderstanding is that balance is the same as mildness. That is not true. Both a light, crisp white wine and a powerful, tannin-rich red wine can be perfectly in balance, as long as the parts match each other's weight.