Today's topic : What common complaints people make at a restaurant? Which one is the most annoying? Why?

I think there're various things that food tastes , their quality and quantity , the restaurant's cleanliness , unkind employees and so on. = types of complaints, such as
All of them are fine if they're legal and I can understand it. = valid (for acceptable reasons/complaints, we say they are 'valid')
Because, all people of world are different // each other, so it's also // different to feel about something. = people all over the world ; to ; we ; have ; preferences
But, It's a little difficult If they're illegal and I can't understand it. = their complaints are invalid
If we can't understand each other , it really doesn't have any method. = I'm afraid there isn't much we can do about it. ( please do tell what you meant by this phrase)
I say summary, i'm most annoyed to ignorant people. = To sum, ; by