What’s Hot: Shows the most popular, publicly shared posts.
Ripples: Shows where shared posts originate from.
What they have in common? Data visualization. Cool for people interested in that, questionable for the general masses.
What’s Hot looks like a more successful feature, but I can’t really tell since I (ironically) got this message:
Nothing here right now. Check out the Google+ search stream to see what people are saying about Google+.