For this factor, I feel that I do not need to write since everyone knows how important it is. Having a proper and unique content with proper grammar is going to make a big difference. In this regard, i would like to say:
1) Each landing page on your site need to have a proper and unique content. That means, your content must not be copied from somewhere else.
2) Try to define, and talk about the main keywords in your landing page. More does not mean that you define or mention it 100 times. As long as what you are writing will not sound silly and annoying for you when you read it, it is going to be fine also when search engines read it.
3) Read about copyrighting. It is the way people write for search engines. I will blog about SEO copyrighting later.
What is a landing page? It is a page that you write to target one or few keywords. It must be also usable by normal users.