I am interested in a plugin that can do instant ajax updates when someone creates a post or makes a reply, so instead of refreshing the whole page, it’s instantly there.  There is a forum software called Xenforo at http://www.xenforo.com which makes heavy use of this feature and it’s great and fun to use.  It really encourages activity because it’s so quick when posting and people like that so they post more.  I think this would be a very popular Simple Press plugin.