Nice one. Great ideas. I think you raised some good points especially regarding newbies. They may not be a great demographic to market to. Since they are new they may be just trying to figure out the platform and may not really understand the long term contest idea. A lot of people join blogging platforms with a specific idea for how they will maintain their blog as well. As you pointed out its probably best to market to active current users. Smart!
Your other points were great as well.