Acceptable to send back: There are foreign objects in it
If there's a single hair in your food, you might as well eat around it. If your meatloaf looks like it's wearing a bad toupee, send it back. And if you find a bandage, we're pretty sure you're allowed to walk the food into the kitchen yourself and throw up on the line.
Unacceptable to send back: You put a foreign object in it
Just because your watch fell off in your lasagna doesn't mean you get a new dish. Or a new watch.
Acceptable to send back: You got a warm salad
Seriously, there's nothing grosser than a hot plate of room-temp ranch poured all over warm, wilted tomatoes and lettuce that sat underneath a heat lamp for an hour.
Unacceptable to send back: You let your food get cold
If your food arrives cold, you've got a problem. If you let it sit there for 10mins while you fielded a phone call, that problem is yours and yours alone.