A dilemma for many people when eating out is what justifies a diner in sending a dish back to the kitchen. Tolerance for imperfection is as varied as opinions about what exactly perfection is in the first place: for me, a piece of roasted meat may be hard and dry while my husband will judge it to be firm and moist.
I eat out at least three times a week, with two of those outings being review meals. When I'm doing a review, I never complain if the food is bad (and trust me, I've been served some truly horrible things). For one thing, I'm trying to avoid bringing attention to myself, and also because I have a better outlet: my opinions about a meal appear in print.
Even when I'm dining out for pleasure, there are only a few circumstances in which I would send food back. The obvious one is when I receive the wrong order (although if the food looks good, I might keep it). Another occasion is if the food is incorrectly cooked: for instance, if I order a rare steak and it comes out medium, because I can't abide overcooked meat. I'll also return a dish if the food is "off" in any way: if the shrimp are mushy (which means they're old), the lettuce in a salad is brown and wilting, or the fish smells of ammonia, which means it's starting to decompose. And I also might send back a dish if I find foreign objects in the food: I've been served (inadvertently) a piece of plastic and the top of a ballpoint pen. A strand of hair? I usually just pull it out and set it aside.
I've never sent back a dish just because the combination of ingredients isn't to my liking, although I know people who do. I read menus carefully and I can tell that baked chicken breast with peaches probably isn't going to be to my taste, so if I try it and it's bad, it's almost as much my fault for ordering it as it is of the chef who conceived such a combination.
If you do reject a dish and request another, take into consideration that doing so is going to require extra time for the kitchen to prepare the food, so you need to be patient and wait. And if you send back a dish for any reason, be as polite to the server as you would be when dealing with anybody else.