One example of a huge cloud-based privacy problem: Google Analytics, which I discovered by using NoScript.
So, google knows about me even when I don't use it. Even when I am explicitly avoiding it, and use DuckDuckGo instead (I do, by the way). Your "benign blog" which uses this service is hurting my privacy big time. I'm okay with you knowing that my computer is visiting your web site. I'm not okay with third parties such as Google, Facebook ("like" buttons, anyone?) or some advertiser automatically knowing that as well.
Same problem with Gmail. They look at and analyse your emails to send you those targeted ads, remember? You pay with your privacy to use it. Which may be fine. But you also pay with my privacy, each time you send me an e-mail, and each time I send one back. That is definitely not fine.
Personally, my heuristic for privacy is this: By default, don't sacrifice your privacy. Think twice before you do. And never ever sacrifice others' privacy without their explicit, informed consent.
We're quite far from that.