If they don't take care of you and you don't come back, then they've lost your business. Also, don't wait until you get home, then dash off a nasty letter. While that tactic will usually get you a free meal, the chefs we talked to prefer to hear about complaints immediately. If you're not happy, say something. If you wait and write a letter, it's too late to fix it.
Reservations are one of the stickiest issues for restaurants and their patrons.
That's the kindest thing they can do. It means they were actually thinking of you, rather than just themselves. Some patrons make reservations at three or four restaurants, then neglect to cancel the ones they don't keep. Most restaurants will save your table for up to 15 or 20 minutes before giving it away. If you show up a half hour late, however, you may be out of luck.