I really like the recent additions. Keeping private discussions as simple as possible, that's my aim. People are used to the traditional ways of writing private messages, which used to be at one clear designated place and only there, so forcing a certain tag (e.g. Mailbox) is great.
I would like to go further and have the option to restrict private discussions to two recipients, the one who initiates a private discussion and the one who shall receive the message. The former is myself, the latter is selected by starting the private discussion via the user's profile page. This way we wouldn't even need a dialog to specify recipients. And instead of showing "2 Recipients" we could simply show the adressee's name, as we don't need to be informed about our own name (exception: if we are participating as a member of a group).