To decant or not to decant: that is the question:
Whether 'tis nobler for the wine to breathe?
With apologies to William Shakespeare, decanting a wine before serving can definitely affect the wine and your enjoyment of it in three ways.
First, if it is an older vintage wine or even a somewhat younger but unfiltered wine, there is the possibility of sediment in the bottle. Master Sommeliers are practiced at the art of decanting a carefully cellared bottle to reserve the smallest portion of the wine and the majority, if not all, sediment in the original bottle. While you certainly wouldn't enjoy drinking the sediment, remember that it was originally and naturally suspended in the wine much like powdered cocoa in chocolate milk. Sediment itself doesn't imply the wine has gone bad; it is just no longer "in solution".
A second reason to decant is for presentation. Whether dining at a formal restaurant or when entertaining guests at home, the process of decanting -- and the elegance of the decanter on the table -- can enrich the ambiance of the evening. It may also remove the pretense or label snobbery when your oenophile guest is allowed to assume the merlot in the decanter is a Chateau Petrus (approximate retail, $2000) rather than the L'Ecole No. 41 merlot you served (approximate retail, $30).
The final reason is that decanting a very complex, very young or just very tannic wine will soften the blow to your palate. In the simplest description, decanting allows oxygen to work its magic on the aromatic compounds, or phenols, and the tannins. The unfolding chemistry in either the decanter or, if you are truly patient, your glass will reveal many more subtle aromas and flavors than quaffing straight from the just-opened bottle. As an example, a 2001 Mauro Molino Barolo is still very young and will be difficult to fully enjoy before 2009. But, with an hour or longer to open up in a decanter, you will enjoy floral nuances, a smoother mid-palate and silkier tannins -- all elements which would be overwhelmed by the initial, powerful notes of earth, alcohol and tannin – that would not be evident if consumed straight from the bottle.
Salute!
-Steve