NOTE ALSO: F9 will select the Attribute properties. If you don't have an attribute selected at the time, it will not select one. If you do, it will open with the selected attribute presented.
I would have thought it more orthogonal to have a selection mechanism and an action mechanism and apply it universally.
For example:
Select what I want to look at.
<Enter> open the properties. <Alt-Enter> opens the container (if applicable). <Ctrl-Enter> opens the list of which this is an element. <Ctrl-Shift-Enter> opens the collections of which this element is the container.
Thus if I have an attribute selected, <Enter> opens the attribute, <Alt-Enter> opens the Class, <Ctrl-Enter> opens the attribute list (at New) <Ctrl-Shift-Enter> opens Constraints
If I have Class selected, <Enter> opens the Class, <Alt-Enter> opens the Diagram properties <Ctrl-Enter> does nothing (or perhaps gives a list of Classes (or Elements) on the diagram for selection), <Ctrl-Shift-Enter> opens Detail Tab.
I'm sure you get the picture...

The essence of object oriented design (in particular the
selection - action paradigm used in MS Windows) is to provide orthogonality in the user interface.
Orthogonality, orthogonality, orthogonality TM 
Paolo