It occurred to me that the line "Well, that's your opinion" is a good variation on "What an interesting assumption." You use this line when someone makes a rude statement in such a way as to suggest it is irrefutable fact. For example, a pushy religious zealot says, "All [members of X religion] are going to Hell." You reply, "Well, that's your opinion" and leave it at that. The response subjectifies their statement, which is, after all, nothing but an opinion, even though they've been trying to present it as fact.
Actually, I got this line from the movie Inherit the Wind.