When hitting Alt+ENTER at the end of a bullet list item, a new item is inserted below the current line. However, there is an evaluation of the line spacing below the list which can result in a “double spacing” of list items, which I personally never want.
In org-list.el, the function org-list-separating-blank-lines-number appears to do the logic relating to this, it called from org-list-insert-item.
My suggestion is that a customize variable be exposed allowing the user to decide whether to have this behavior or not.