Make the voting buttons ('up' and 'down') visible to Guest User but when they click on them, redirect them to the registration page. The idea is that this will show the interactivity of E2 and get more people involved who wouldn't be otherwise (in theory).
Separate the message inbox from the chatterbox. The current system is pretty unintuitive to new users and the fact that the message inbox is displayed with the chatterbox doesn't help the matter. One way to fix this would be to make the Message Inbox its own nodelet to prevent the confusion. This could be supplemented by putting a link to the Message Inbox in the Epicenter and, if possible, tracking whether a message has been read or not. If any messages are unread, the text on the link for 'Message Inbox' would change to red (or some other eye-catching color) with the number of unread messages next to it in parentheses. For example, if I had 6 unread messages, the link would show up red with the text 'Message Inbox (6)'.