|"Commonplace book mid 17th century" by Beinecke Flickr Laboratory|
They reason commonplace books don't quite make journal status is because they were almost never used to discuss the entries. I might throw a recipe in my journal but I'll talk about it, why I like it, what I'd change about it and so on. My journal is a place where I discuss things with myself.
A commonplace book would be a place to throw an idea so I won't forget it or to stash a funny quote that I know I'd like to share on Facebook later.
There's more about these interesting artifacts from the past on Wikipedia. (It's where I got that wonderful picture!)