There are a couple of factors that define a project. Usually you have three or four of them. So what are those?
Why are they key? They are highly coupled (think rubber bands) and modifying one of them changes the other dimensions as well.
You say this is common sense? IMHO you are right.
However, if you try to communicate changes in one dimensions (aka change requests) and mention the consequences for the others, even seasoned project managers seem to forget this…