There are many features available in Qlikview.
First thing you need to understand the customer and the nature of there business, this will help you to choose the proper features of qlikview to demonstrate them.
I will suggest you to go through the demos available on qlikview site.
Allong with all the above make sure you show the special icons which makes it posible to send object very fast to Excel, your cliboard or to a printer.
An other feature that I noticed that a lot of customer really like it the report editor.
An other tip: explain/show how it works. How fast and easy information can ben shown.