Used to be that when you emailed someone five times from Gmail, that recipient automatically got added to your contacts list—but no more. The Official Gmail Blog explains that now message recipients who you haven't explicitly added to your Contacts list yourself go into a separate "Suggested Contacts" list. Suggested contacts still show up in the To: field's auto-complete, but still separate "people who you cared enough to edit or add by hand" versus "people you replied to a few times." The update will be retroactive, too:
As part of this change, we're moving previously auto-added contacts back into Suggested Contacts. Only contacts that you've edited, imported or added to a group will remain in My Contacts. This will provide everyone with a clean slate and, we hope, a better point for syncing contacts with mobile devices (for example with Android). We'll be rolling this change out to everyone over the next few days.
Minor but helpful upgrade. What other updates do you want to see to Gmail's contacts? Shout 'em out in the comments.