It seems Google Apps accounts are opted out by default, but Gmail and other regular Google accounts may be opted in by default.
Still, they have made it very clear how to turn it off, and you would still need to comment, +1, or follow something for "Sharing" to kick in. I can kinda see how it's a nuisance, but they are being very up-front about it and making sure all user are notified via several notification methods.
I'm more bothered by the half-assed attempt to tie my Google account and real name to things like YouTube. Those prompts are (still) down right infuriating (and buggy, a few times I thought it might have change my YouTube account name).
This however, is hardly a blip for me. (Although to be fair, I use an Apps account).