[kalistoga]

What you described (recommended posts within a site), as well as someone else mentioned below (recommended posts from other sites) are already out there. If you ask me if the idea sounds novel, it is not. Just visit Techcrunch and take a look (For example, http://techcrunch.com/2011/11/28/paypal-cyber-monday-mobile-...)

You will learn if you start doing more market research, but I've found it's either of the two for your initial idea: 1. There is someone else doing the same thing out there; 2. You haven't done enough research.

Even if something similar is NOT out there after all your research, it is highly likely that it has been tried before and failed (and that's why you can't find them). Even these products you can dig up more if you do intensive research via google. You'll learn a lot while going through this process. Just don't be naive and think your idea would be novel when you haven't done any market research yet.

The key is in absorbing as much knowledge as possible in the field you would like to tackle, and later to be able to say "Oh I came up with this idea, and I am 99% sure this is a unique idea that hasn't been done before, because I 'know' all that's been done before"

By the way, Posterous started out as a niche product. The platform may be trying to do everything nowadays, but in the beginning, they were tackling email to blogging niche, which didn't exist before. Everyone thought blogging should be done by signing into your wordpress account, typing a title and content and reading the preview, and then posting.