For example, as your potential customer, I think your landing page would ideally be:
* About the exact same thing as the keyword that I searched on
* About the exact same thing as the ad I clicked on
* Quick to load - and devoid of time wasting fluff
* Easy to navigate in case I want to browse and buy other stuff, check your contact info, learn about your shipping policies, etc.
* A page that immediately conveys the feeling that I can trust your business with my credit card number
As I said, this is just my $0.02. But when all is said and done, a landing page is about showing your potential customer that you have what they were looking for, and also making them comfortable with buying it from you.
AWA