PriorityInbox, a feature designed to push a user's most important e-mails to the top of the inbox and leave them there, was scheduled to be rolled out at midnight ET Monday.
And you need to proactively click around to other tabs to see less-important messages, so the Primary tab feels more streamlined than Priority Inbox's stacked-inboxes approach (the latter feels a tad cluttered, at least to me).
Conceptually, this is simpler than PriorityInbox, since Gmail is only trying to sort messages into general categories rather than figure out which people are more important to you than others. (I've been using the new version for a few days, and the sorting seems to work really well).