I'm not sure about affective (did you mean "effective"?), but we can definitely identify a common, even standard, design pattern for one-product websites.
Here are a few examples: Square, Doxie, Feedly, Highlight, Dollar Shave Club.
As far as I understand, the main principles for one-product websites are:
- Product in front - These websites use big images and large fonts to promote their product. The product (and its price!) is in the prospective buyers' face. They don't have to guess or dig it up.
- Focus on the user - These websites don't write about features nobody cares about. They're also not filled with marketing blabber (e.g. "It's the best!"). Instead, these websites focus on the user and how the product they offer can help with their goals/problems/needs. For example, in square's website, the tagline is "Start accepting credit cards today".
- Strong CTA (call-to-action) - These websites are clear about what they want the user to do - buy their product or sign up for their service. All paths lead to it. You can either do it straight away, or read more about it before you do. To achieve this, they use big buttons with strong colors and clear labels: "Sign up", "Buy [product]", "Do it", "Download for free". Of course they cater to other needs as well (customer service, company info etc.), but these are all diverted to the secondary navigation elements.
- Clean, minimalistic aesthetic design - Visual appeal increases perceived credibility (6th guideline in the Stanford Guidelines for Web Credibility).
These are of course common characteristics I gathered from analyzing this type of websites. Each case is special, and you should design the website to suite the specific business requirements and target users' needs.