Actually I think the fact that reddit enables pseudonyms makes it quite different. I don't particularly want to share the fact that I'm super into weird animes with my colleagues, and reddit enables this, google+ not so much.
What if you're a recovering or current addict, and your support group is on a subreddit? What if you're advising people considering opiates to not follow your path, based on your own life experiences? That's not likely something you want to sign your name to.
What about domestic abuse victims who want to support each other? Victims of sexual abuse as children? Parents of children who have been abused?
What if you're into something that's would really set people off, even if it's something consensual? Consider what's happening over in Drupal-land - regardless of your opinion of Goreans, BDSM practitioners, etc., what if you're just into something that your local community or coworkers find anathema?
You can't fool the machine but at least you can hide that you like anime from your co-workers and relatives.
I know people that own like 10 Google accounts between their YouTube and other services. You can't trust Google to not expose you (like they did it with YouTube account merges, Google Plus and other instances).
Reddit allows compartmentalisation of interests. G+ actively prevents that.