Forums are mostly known for the number of impressions and unique visitors (mostly impressions)
You can sell away banners on CPM basis or just set a price. Text Links on the header also serves as a good sign of advertising. Currently I am making more in banner and text link sales rather Adsense. I am using both actually. I am showing Banner ads on the header below every first post of every page on a thread. I am showing text links below navigation bar and in the footer. I am earning a fair amount of money from my forum even though adsense gives me way too less CTR. Adsense isnt the only king here 

yes, i am not sure how they assess your site for text link ads, but i have had a few popular sites turned down by them too.  i think they consider age and alexa rank (both of which have little to do with a site's popularity.)