Parsha Question

My mother recently found buried in one of her bookshelves a notebook of mine from 7th grade.  Looking through it, I found the following question, with no answer written down.  For all I know, its very obvious, but I’m not sure what the answer is.

The Pasuk says that Hashem smelled Noach’s korban and was so pleased that he promised never to bring a mabul again.  Why did Noach get such a reward for a simple korban?  Hevel, Kayin, and Adam also brought korbanos, but they didn’t get such a promise.  What was different about Noach’s korban?