For the longest time “pageviews” have been the metric that ad networks look at, but now that’s slowly starting to change to “viewability” (50% of the ad visible for 1 second). There was a recent Google research report showing that >50% ads are never viewed by humans.
This may mean more ad networks will eventually be okay with auto-refreshing ads even if the page never refreshes. DFP’s ad tags already support auto-refresh calls, but using that functionality violates most ad exchange TOS, including Adsense. Hopefully this is slowly changing–for example, right now OpenX lets publishers auto-refresh their ads, provided the page content has “changed significantly”. However, getting into the OpenX exchange requires the publisher deliver several million ad impressions per month, so this won’t work for most folks.
I own two forums with a combined ~6M posts, with hundreds of active users every day, and the biggest obstacle to migrating them to Discourse is the lack of effective monetization. For example, even if I auto-refresh ads using OpenX, there’s no convenient way to insert the DFP code into floating sidebar ad units, or into every Nth post. Yeah, we could write this ourselves, but it’s a lot simpler as an owner/admin to just use something like Xenforo.
Hopefully both the ad industry will shift paradigms and Discourse’s theming support will mature quickly… we’ll see.