If kept for long enough, wine eventually turns into vinegar after being opened. Depending on one's taste for vinegar, this can be a plus or a minus.
The taste of a wine can change noticeably after being opened and exposed to oxygen. Sparkling wines and champagne lose their fizz after opening. This process can be slowed down by limiting exposure to the air, but this solution only delays the inevitable. Wine that is not as pleasant to drink can still be used effectively for cooking.
Fortified wines, as their name suggests, have long shelf lives because of the way they are made. The extra alcohol added to the wine acts as a preservative.