You want to send food back. As a rule, send a dish back only if it isn't what you ordered; it isn't cooked to order (a supposedly medium-well fillet arrives bleeding, for instance); it tastes spoiled; or you discover a hair or a pest. Just speak calmly and quietly to the server when making the request.
Restaurateurs can tell you that keeping diners happy is one big job. They also point out that people are much more likely to voice their complaints than their appreciation. That's not to say that you shouldn't let it be known when the service is slow, a server is rude or careless, or the food comes badly prepared. The restaurant depends on its customers' approval for its livelihood, and its faults can't be corrected unless they're brought to the management's attention.
Make a complaint quietly, without attracting the attention of other diners. Speak first to whoever committed the error. If he makes no effort to correct the situation, take your complaint to the manager or captain.
Rudeness and laziness might be reported, but don't confuse them with the inability to serve too many people. Often, a server works as hard and fast as possible but still can't keep up with patrons' requests. If this is the case, you could still complain to the management, but be careful not to put the blame on the server, who's undoubtedly no happier about the situation than you are.
If after making a legitimate complaint you receive no satisfaction at all, you might reduce the tip (or, in the most extreme cases of bad service, leave none) and avoid that restaurant in the future.
On the other side of the coin, appreciative comments and a generous tip are more than welcome when you're pleased with the service. While tips are expected, comments like "The food couldn't have been better" or "The service was especially good" are a pleasant surprise and mean a great deal to someone who is trying to do her best. The management will also appreciate hearing from a customer who is satisfied and doesn't hesitate to say so.