What''s new uses the "modification date" for ordering of the content. When a piece of content is created, the modification date is the same as its creation date. After that, each content type has slightly different behavior for modification date.
The modification date on a thread will be the date the last message in the thread was created. This means that a new message being post to a thread should push that thread to the top of the Whats new list.
The modification date on a document will be the last time that document was edited. The What''s new list only shows published documents, so a newly edited (and published) document should move to the top of the list.
Blog posts are very similar to documents in that the modification date will be the date the post was last edited.
In the future, we plan to make the Whats new more of an activity view, rather than a view of content. It will display things like "User X commented on document y" and "New poll created in space Z", etc.