As beforehand announced, Gmail added a setting that lets you disable automatically saving email addresses to your contacts. Go to the Settings page, find the section "Create contacts for auto-complete" and you'll notice that the following option is enabled by default: "When I send a message to a new person, add them to additional Contacts so that I can auto-complete to them next time". Now you can stop this feature and select "I'll add contacts myself".
This is one of the features from a long changelog of small improvements. "Refresh" is now a button, the keyboard shortcuts guide is now obtainable even if keyboard shortcuts are disabled (just press Shift+?), Gmail shows more helpful warnings when you leave out the "." in ".com" from an email address and there are fewer warnings when you respond to a message in the Trash.