Wow! A very clear explanation :) Thank you Nina!

Yes, "No worries" can be used to mean "Don't worry (about it)." It can also be used in place of "no problem" or "that's alright." For example, person A accidentally steps on person B's foot: A: Oh, sorry! B: No worries. / Don't worry about it. / No problem. / It's ok. / That's alright. It's not a formal way of speaking but I've heard it used in a variety of settings. It originated in Australia and is used to convey a relaxed attitude toward life. (Wikipedia, 2014) I think whether it's used for important or unimportant issues depends largely on the speaker's perception of the situation.