When we’re speaking, we also have facial expressions to help us communicate tone:
And hand gestures:
And if we’re having a conversation, people can interrupt us and ask for clarification:
When we’re communicating via the written word, basically all we've got to work with are words. Words are great; we can communicate quite effectively with words, but, as a passionately sarcastic person, I am glad that we can use italics to show emphasis and quotation marks to show irony.
Emphasis:
I have to have those shoes.
Irony:
Is this another one of your "great" ideas?
Quotation marks and italics are not interchangeable.
I just graded an essay in which the student wrote:
Shelly and I always have "meaningful" conversations.
From the context of the essay, I could tell that my student was being sincere: she and Shelly did, in fact, have meaningful conversations. I think she used the quotation marks in an attempt to emphasize just how meaningful their conversations are.
However, by using the quotation marks, she communicated just the opposite: that instead of exchanging theories about the meaning of life, she and Shelly sit around and talk about superficial, petty, vacuous things like hair, make-up, fashion and celebrity gossip.
By the way, does anyone know who designed Kate Middleton's wedding dress? I wonder if she's going to wear her hair up or down. I hope she doesn't overdo it with the make-up.