One of the last remaining rituals of restaurant services is the pouring of a small sample from a bottle of wine ordered for the host of any party to try. I bet the great majority of those who do the trying (and indeed of those who do the pouring) aren't too sure of the point of it, which is for the person who has ordered the wine to check the temperature and whether it has a fatal flaw. I quiet often find in restaurants that reds are served too warm (so I ask for an ice bucket) and whites can be too cold (so I ensure the bottle is left out of an ice bucket).
As for the flaws fatal enough to allow you to send a bottle back, the principal one is that the wine smells too mouldy to enjoy. The most common reason for this smell, often called TCA by professionals (short for trichloronanisole, the chemical compound responsible), is tainted cork. Such wines are often called 'corked' or 'corky'. The problem is that the level of TCA vary considerably. This can result in some rather heated discussions between waiters and customers, but you could point out that, unless the wine is very old, the restaurant may well be able to return the bottle to the suplier and get a refund. One common side-effect of TCA is a lack of fruit on the palate. Note, however, the important fact that you do not have the right to refuse to buy an opened bottle simply because you don't like the taste.
Two of the most infuriating habits of some waiters are to fill up your glass more frequently than you want, and/to fill up your glass to such a level that there is no room above the wine for the all-important aroma to collect. You will be doing all wine drinkers a service if, politely but firmly, you make your feelings about these practices clear.