I think the most important bit is 'The best examples of this business model are when the customer implicitly understands why the paid service has to cost money

I think the most important bit is 'The best examples of this business model are when the customer implicitly understands why the paid service has to cost money. More storage costs for photos or virtual storage are good examples.' If your customers are paying for their 'usage', but it don't understand how it is actually a cost to you, they can feel ripped off. I'm surprised he didn't include Angry Birds in the list.