"My Contacts is a place to import, store and view all of the contact information that's important to you. You can also create your own groups of contacts to easily email many people at once. We add people you've emailed to Suggested Contacts. You can move contacts from Suggested Contacts into My Contacts at any time."
There's an option to automatically move suggested contacts into My Contacts if you frequently email them, but the previous behavior doesn't seem to be an option.
I don't have this new version of the contact manager, so if you notice other changes, post them in the comments.
Update (July 16): Gmail's blog announces the new feature, explaining that all of the addresses show up in auto-complete. The contacts are now separated in two sections: My Contacts (the addresses explicitly added to Gmail) and Suggested Contacts (addresses automatically added by Gmail). "By default, Suggested Contacts you email frequently are automatically added to My Contacts, but for those of you who prefer tighter control of your address books, you can choose to disable usage-based addition of contacts to My Contacts."
{ Thanks, Brian G. }