I got a message from Microsoft saying that my AdCenter campaign got put on hold "as part of our editorial review process" for not following content guidelines.
Then when I checked what the problem was, it seems that MSFT didn't like me using quotation marks (" ") in my ad headlines. I read through their editorial guidelines and didn't see this specifically forbidden.
Google has no problem with the same ads and I wonder if it was just some new overzealous employee at MSFT that day... Has anyone else had similar experiences with AdCenter?