I don't think pay per click affiliate programs are the best revenue makers either. I mean, obviously, selling a product or service and doing ecommerce is going to have the most direct ROI associated with it. ie you spend $X to get a visitor, you make $XX when that visitor buys. You just made $XX - X money. Essentially, it's pretty low risk.
Content based sites which rely on advertising revenues are a bit harder to discover their hidden ROI. You could spend $X to get visitors to your site, and you have no idea whether they're sticking around, for how long, whether they revisit often, or just made a one time visit. Advertising money you spend today might not be noticed until next week, for example, when they come back for their third or fourth visit and start to browse the site a little bit more.
For example, with an ecommerce site, you can use Google AdWords or something to pay for visitors definitely interested in what you're selling. With a forums site, you can use Google AdWords to pay for visitors interested in what your site is about - but it's completely hit or miss whether they're a forum junkie

I guess that's where really good ad copy comes in! .... But even with that, you have no idea whether their personality meshes well with your community. I think there are just many more factors that come into play as opposed to => Interested in buying this product. It has a very competitive price. I'll buy it.
With a forum or blog or other content site ... you're essentially trying to convince the person why they should give up one of their most valuable assets - their time!