1) Creating fake pages. Yes, it's prohibited here, too. Big shock. Moving on.
2) Running contests. Ooooh, that's a new one! You cannot run contests, sweepstakes, offers, or coupons directly on your Google+ page, but you can display a link to those promotions that leads people offsite.
3) Aggressive Circling. That's a ... weird phrase. But much like you can't aggressively follow and unfollow people on Twitter without getting flagged, you can't Circle a ton of people on Google+ without punishment.
4) Altering or adding +1 buttons where they don't belong. Similar to the Twitter badge rule, you can't, say, superimpose the Google +1 button on an ad. It's a misleading way to garner clicks.
5) Keyword stuffing. Yes, it can happen here, too! Because Google+ is so closely tied with organic search, the penalties are just as stiff. If you're trying to rank for a keyword, stuffing it into every Google+ update is not the way to do it.
6) Marketing regulated products. If you're marketing in a regulated industry, such as alcohol, tobacco, medical devices, fireworks, pharmeceuticals, etc., you cannot use Google+ to market those topics.
7) Letting your page go dormant. If your Google+ account is dormant for more than 9 months, Google can snatch it right back from ya.