"당연하다 vs 물론이다". The key difference is the moral undertone of 당연, which 물론 does not have. - 당연하다: 당(just, proper, right) + 연(like so) => be as it should be; rightfully so. 당 implies a moral judgment, a conformation with a broad expectation. - 물론이다: 물(not, forgo) + 론(talk, debate) => of course; needless to say. All 물론 says is something is a matter of course, a strong affirmation between the speaker and the listener, free of any moral judgment. Examples: - 약한 자를 돕는 것은 당연한 도리이다 - It is what's expected of us to help the weak. - 학생들이 이렇게 반응하는 건 당연하다 - Such a reaction by the students is as expected. - 나는 물론 후자를 택했다 - Needless to say I went for the latter choice. - 차는 물론(이고) 집까지 팔아야 했다 - Even had to sell my house, let alone the car.

당연하지 implies that what the other person just said is a matter of course, reasonable, fair, etc. 물론이지 is more like,,, "Sure, I'd love to." It is often followed by someone's request questions like "Would you please ~~?" It is to show that you are happily willing to do what the other person just said.