OneNote can do outlining that can collapse/expand.
This is a critical feature to me and is missing from Evernote.
Here is a link showing how OneNote does it, it's pretty clean (screenshot from link shown below).
I'm sure there are other ways to make outlining even better, but starting with this simple implementation would be a great improvement.
I would hope that the state (expanded/collapsed) would be remembered from one session to the next.
To Evernote Staff: Where does this feature fall in the priority list for evernote? Is this on the roadmap?