Billyboy, As I believe you mentioned earlier sometimes the dish is question isn't necessarily something new. It could very well be a familiar food such as meatloaf that is prepared differently, uses different filler etc and as a result it's something I don't care for. Should I just "grin and bear it" pay for it and go away unhappy?
I really don't have the funds to pay for two meals in order to get one I can enjoy.
I don't have the money to pay for two meals, either. Thus if I order something and find it not to my liking for whatever reason that does not involve it being made wrong, I do grin and bear it. Because it's MY fault I don't like something. The kitchen prepared it the way they normally do, using ingredients they always use, and presented it according to how it says on the menu. If I'm not sure I'll like something to the point where if I don't like it I won't be able to eat it, then I choose something else from the menu in the first place. It is at best inconsiderate to expect to be given something for free because what you ordered doesn't taste good to you.
Hell, that happens at my house sometimes! Probably three nights out of seven I'm creating something in my kitchen without following a recipe. Sometimes I lose, often I win. But on the days I lose, I suck it up and eat it anyway, because I can't afford to throw the food away. I figure the same applies to a restaurant.
The one exception to this is food allergies. In the past, it has not been uncommon for restaurants to use cantaloupe slices as a garnish instead of parsley and an orange slice or whatever. I am highly allergic to melon, and cannot eat anything on the plate if it's there. Since prices have gone up, it's rare to see it, so I don't always remember to ask if they use it. If my plate comes out with a slice of melon on it, I apologize profusely and explain the deal. I always offer to pay all or half of the cost of the meal I can't eat, and I have always been told that's not necessary. And trust me, I know they're not taking the plate back to the kitchen, removing the melon, and serving me the same plate. I would have a reaction, and it wouldn't be pretty.
But simply not liking something? No way would I expect them to replace my meal for free.