A common marketing mistake is to assume that your reader knows what you're writing about. Especially with new websites or online communities, such as LinkedIn.
An even bigger marketing mistake is to assume your reader understands the benefits of your subject. I thought about this after reading an e-mail from another company. They promote workshops that show people how to get started with and utilize LinkedIn effectively.
Through my experience and looking at other marketing materials, I've learned that you won't market yourself, products or services effectively if you assume your reader has a magical ability to read your mind and know what you mean.
As a rule of thumb, always assume that you're writing to a kid in middle school. If what you've written is clear enough for her to understand, you've probably done a good job of communicating your message.
The easier your message is to understand, the easier your job will be to sell more of your product or service.