Ok, thanks. I prefer to block everything by default, so for me per-site basis is not very convenient. In the case of Disqus: on most websites I don't read the comments and I prefer to block all the requests (mostly 100+ if you include the gravatars).
With Ghostery I just block everything, but this is also not perfect because, as you mentioned, you sometimes don't see all the content. It would be nice if the add blockers could block content in the way Chrome blocks plugins: everything is disabled, by you have a placeholder which informs you that something was blocked, and when you click on it, it loads the content.
OpenX is indeed blocked (I checked it with the chrome dev tools), but it doesn't show up in the Advertising category, e.g. http://www.openx.com/.