Reshare and Publicity
Many of my earlier comments about discovery/serendipity have been dodged by one thing I didn't anticipate. A huge portion of the content in Google+ is public, like Twitter. This means that there is a public sphere and a very private one. Nice way for Google to be in the game. For me, it's becoming hard to find the private content now (and this is not necessarily good).
Re-share also has some significant benefits. I'm not a fan of the re-share presentation (it should be more compact and integrated), but I get the value of it. To take a public conversation into a small group and talk about it makes a ton of sense. Already, some of the huge public threads have become incredibly unwieldy. I don't care about 500 public comments, but more about what my friends have to say.
The Plaxo Curve
When Plaxo first crawled our Outlook address books back in 2002 and encouraged everyone to invite all their contacts in one go, it was novel, because nothing had ever spread so fast. I recall it took Aol's Instant Messenger nearly 12 years to grow to 90 people per buddy list. But after Plaxo, it took this next generation of social networks not long at all. (My Google+ account has this many people, already.) Since then, it's become necessary for each new service to plumb your email contacts and IM contacts and make friend of a friend suggestions, in order to grow quickly enough.
If you think of it that way, the invite policy of social software results directly in a growth curve. Tagged.com took it too far, spamming friends without even asking your permission. Google's prior effort, Buzz, mostly confused users by not giving enough control. Facebook takes it right up to the line and stays on the side of user-controlled politeness, with a great Friend Finder that can login to a bunch of services.
But especially given the limited number of people on the service, Google's "Plaxo number" is very high indeed. People I don't know are adding me, sort of like Twitter. And I already have >100 people in my circles, despite being on the service for only a few days. Google isn't logging into as many services as Facebook, but they're making the contacts they have count for a lot. Never has adding people been quite so fun.
This bodes well for growth, and with Google's sorting tools, that growth is manageable too.
Based on promises that you would be able to share any entry by email, I uploaded a video and shared it to 20 people who haven't got Google+ accounts yet. They all got "permission denied" errors and emailed me about them. So it goes with a field test, I guess.
Integration and Speed
The integration with google.com navigation is incredibly compelling. Every search result tells me when Google+ has new stuff for me. I look at that top bar on Google more times per day than my email. Seriously, this is almost as good for Google as building Google+ directly into the browser.
A related bit: Google+ is incredibly fast to load, so I'm never afraid to popup the notification window or clickthrough. For me, Facebook's mobile app in particular has been taking >10 seconds per pageview, which gives me pause if I want to click through an email from my phone, or not.
Limited sharing a good match for photos
My usage of Picasaweb has been declining, but I expect Google+ to bring it back. Sharing by email was laborious, and the previous "groups" interface was daunting. It was becoming too easy to post a few photos on Facebook instead.
That said, I believe the Circles UI doesn't scale as well as it could.
I have too many people in the "friends" bucket, just like Facebook. I started adding "Family" to "Friends" also, halfway through, so I could just share with one group most of the time. Understanding which users are in multiple groups is often confusing. Knowing who you're sharing could be better, but today is not so clear: you have to be really precise about managing people in order for the experience not to devolve into an experience exactly like Facebook.
There's mention of an upcoming API. I can't wait.