To tag friends in posts, type "+" or "@" followed by their name. You'll see an autocomplete drop-down menu show up as you type their name, which presumably includes people in your circles and extended circles.
Your friend will be notified they've been tagged in a post, and post visibility will automatically be set to just that person. Don't forget to add more circles and friends (if you want to) before sharing.
Oh, the irony. To share a post with your Twitter or Facebook network, use the Extended Share for Google Plus Chrome extension. Upon installation, you'll see a new option ("Send to...") below each post in your stream.
Here's a nice feature for any on-the-fly photo editing. Go to your photos (accessible via your profile), select a photo. Click Actions > Edit photo, and you'll be presented with several photo filters. Scroll through other photos in the album for consecutive editing.
With this setting, you can let people e-mail you directly from your profile. Head to your profile, then select "Edit profile".
Below your profile photo, you'll see a grayed out "Send an email." Click it, and check "Allow people to email me from a link on my profile." Then adjust the privacy settings below.