One mistake I often used to make at Justin.tv was offering a potpourri of business models (virtual goods, product placement, chat ads, contests, etc..). I was embarrassed to say Justin.tv would monetize with advertising when clearly that was the only answer. Own the simple business model.
Along those lines, I believe that mechanics design is a distinct discipline separate from both interface design and product management. I believe it deserves its own terminology, principles, processes, deliverables, and bodies of knowledge.