Here's a nuance that hasn't been mentioned yet.
We have the pattern: Statement, tag?
Now, both statement and tag can be either positive or negative. That can change the nuance:
If the statement is positive and the tag negative, or the statement is negative and the tag positive; and the tag is pronounced with falling intonation, the speaker expects the other person to agree:
The earth goes around the sun, doesn't it?
The sun doesn't go around the earth, does it?
Often, this can be used to try to elicit a confession of some sort:
You murdered Mr Brown, didn't you?
You didn't do your homework, did you?
Some linguists actually consider the question mark to be incorrect here, because although the tag is constructed like a question, it is not a real question at all. So sometimes you may see the tag in this case written without a question mark:
Britain is an island, isn't it.
Although I personally dislike this idea, it is useful to distinguish between this case and the next case:
If the statement is positive and the tag negative, or the statement is negative and the tag positive; and the tag is pronounced with rising intonation, the speaker is unsure as to whether the statement is correct, and is actually asking the other person to either confirm the statement or correct it. Because this is a real question, it should always have a question mark:
Sean Connery is from Scotland, isn't he? (Rising intonation: I think Sean Connery is Scottish, but I'm not certain.)
Monday isn't a public holiday, is it? (I don't think Monday is a pubic holiday, but I'm not certain.)
If both statement and tag are positive, this indicates disbelief, surprise, anger or (perhaps less often) a sense of inevitability, or some other emotion:
You're 150 years old, are you? (Disbelief)
It's 10 o'clock already, is it? (Surprise)
You stole the money, did you? (Anger)
And then I suppose it all went wrong, did it? (Inevitability)