Weather & Harvest
Weather is a major factor is determining whether a year is going to be a 'good vintage'. For example, was there enough heat during the growing season to lead to enough sugar? During harvest time, the short-term effects of weather are quite important. To produce great wine, the fruit should be ripe (but not overripe), and have a high (but not overly high) sugar content ('brix'; typically about a 22 brix for table wine). Think of raisins - as the fruit dries, the water evaporates, what is left is the sugary fruit. If it rains just at the point the wine grapes are ready, and before the grapes can be harvested, the additional water will cause the water level to increase, and the brix (sugar content) will go down. That may leads to watery wine and therefore can not be considered as great wine.
Every year the wine grape grower plays a game of chance and must decide when to harvest. Simplistically, if they knew it wasn't going to rain, they would just test the brix until it was just right, then harvest. If they harvest too soon, they will probably end up getting a wine too low in alcohol content - there won't have been enough sugar to convert to alcohol. These wines will be too thin in texture and alcohol content. If they delay harvest, there may be too much sugar, which leads to a low acid content. This also affects the balance and therefore the taste (and the aging potential) of the wine.