What's OK?

Exactly a month ago, J. Wilder asked the following on the ETJ mailing list:

"Looking for a succinct, all-encompassing answer to the question what's the difference between 'That's okay' and 'It's okay'?"

In most of the instances that I could find, they seem interchangeable. There are just a few where it doesn't seem to work. For one thing, if I have done something that you think is wrong or something has happened to me, and I'm trying to reassure you that nothing is wrong, "that's OK" doesn't work. On the other hand, "it's OK" doesn't work in the following situations:

1. Could you zip through my letter and see if it's OK?
2. It's just one of those things. It's OK.

(Note, replacing that with this doesn't fix the problems.) My first thought was that it would be a difference between endophora and exophora, but I don't think this is true since both seem to work when there is an antecedent and both when the referent is external to the words of the conversation.

In fact, I don't think I could consistently explain why it or that is prefered in a variety of situations.

Anybody able to help out?