Which phrase would you choose to replace the ubiquitous "Have a nice day" phrase?
I don't have a problem with "have a nice day." I think it's a nice sentiment, actually, even when just said as rote routine by bored people.
Although there are some situations where common sense should dictate that it not be said. The veterinary clerk said it to my mother on the day she picked up our cat's ashes when I was 18...a cat we'd had for over a decade whose death we were really broken up about. "My condolences" would have been more appropriate.
On a similar note, I was watching a medical documentary a few weeks back and there was a segment in which a doctor was breaking the news to some poor shell-shocked parents that their bright and vivacious 7-year-old daughter had an almost certainly fatal, inoperable malignant brain tumor. I was, of course, as the parent of a bubbly 7-year-old girl, sobbing my eyes out when watching it. The parents quietly said "Thank you doctor" when he had finished explaining the news. He replied, "you're welcome." Not the best thing to say, although I'm sure it was just a learned in reflex to someone saying "thank you," but certainly a reminder that it helps to think before you speak in all situations.