Which kind of problems are described as "That's no rocket science"?

Your understanding is correct. "That's no rocket science" simply means "That problem is easy or straightforward." In other words, you do not have to be a rocket scientist to solve it. It does not really carry any more information than that.

Incidentally "He is no rocket scientist" usually means "He is not exactly dumb, but not too bright either".

In addition to the usages given by Dima there are a few creative implications to using this phrase.

For example you can use it to suggest that normal Joe could do something himself. "Come on Joe just get in there and do it, it's not rocket science."

It is also used in reference to past scenarios that mystify people but all the clues are there and they just aren't thinking straight. You can imply that they should have thought of that themselves by saying "it didn't take a rocket scientist to figure it out ..."