The design of the landing page would depend on its purpose, whether it is to make the visitors buy products or to make them opt in for newsletters and promo offers. Whichever it is, a landing page should generally be designed to make or entice site visitors to do what the site wants it to do.
With a purpose of making sales, the landing page should capture all the attention of the visitor right away. It is important that the landing page looks professional. It should be clean and well-designed so that visitors will take the site seriously with just one look. In designing a landing page, it is important to remember the attention span of site visitors is very short. So, they should be hit by an appealing design right away. Then it should be coupled with informative. The information provided should be valuable to the visitors. A product review or a helpful advice would surely be appreciated by the readers. But while providing such information, the landing page is subtly and yet strongly doing a sales pitch. People are generally more resistant to a direct sell. Instead trust should be formed by giving them useful information first.
Nevertheless, all information should be about the product only so as not to distract the visitors of other things. All information should also match the action required from the site visitors. No other links other than the purchase link or an opt-in form should be found in the landing page. It would help to offer freebies and bonuses with the purchase. Alternatively, they can be offered in exchange for an opt-in as well.