As you said, the key to Google+ is engaging with others… showing an interest in others within your network

Google Plus: Getting Top SEO Rankings Without Top SEO Metrics Posted on April 24, 2013 by Todd Mintz

5 Knockout Google+ SEO Tips

SEO for your Google+ Cover Photo

Auto Awesome and Google Plus Covers Add additional SEO

An Auto Awesome image represents a huge opportunity for businesses and their discoverability efforts in Image search.
In the example below there are a series of 9 images (8 original, plus 1 animated) that can each be optimized for their key search phrases.

I Have Discovered the Purpose of Google Plus!

10 Ways Google+ Will Improve Your SEO

Google+ Profile Optimization

How to Build the SEO Power of Your Google+ Profile or Page
Written by Mark Traphagen / March 28, 2013

Based on what we’ve observed above, I would suggest the following strategies to build the search authority of your Google+ profiles and brand pages:

Follow the techniques I outlined in my previous two Windmill Networking articles (linked in the “Google+ and Web Influence” section above) to build relationships with others on Google+ who are influential in your field. As they begin to see you as a useful and helpful authority, they will begin to share your content and recommend you, creating connections that will build your own authority.
Pursue opportunities to earn or build links to your profiles/pages from authoritative web pages.

Expanding on #2, here are some suggestions for link building to your Google+ profiles and pages:

Here is a list of social media sites that allow a dofollow link in your profile. You might want to consider making your Google+ profile your hub link for such profiles.
Include a profile link wherever possible in guest posts you write for other sites.
Use Google Authorship, which always creates a link back to your profile.
Connect your web site to your Google+ brand page with a verified rel=publisher connection.
Frequently publish blog-quality or newsworthy content directly from your profile or brand page. In addition to Google+ reshares you’ll attract, web sites and blogs who find the content interesting can link to it directly. (You can grab the direct URL of any Google+ post by right-clicking on it’s time stamp.)

Conclusion: As I’ve been telling you for the past year in these articles, Google+ affords businesses opportunities to expand reach and influence online like no other social network. It is a doorway into Google of a kind they rarely open so freely. By building a powerful and extensive Google+ network, you have the opportunity to directly affect the personalized search results of thousands of people in your extended network. But as you build the authority of your profiles and pages, you also have the ability to gain ranking positions in search that can be seen (potentially) by anyone and everyone, even if they aren’t in your network or even on Google+.

Complete Guide to Google Plus Posts and Photos