What it the most important in a demo presentation ?

The Features you show them in order to meet their business requirements are important, no doubt about that.
The Value that your software, your commitment, your solutions and so on ... can bring to them does matter a little more than only features.
Finding out the main message you want deliver to your customer and demonstrating your Unique Business Value to the customer's eyes, is good.
But above all, you need a Story ! before working on a demo preparation you need to find a story that help you to deliver your main message., the one message.
The message is the goal and the story is the path. But how to create a story ? it 's not easy !

Euh ...you may have some brainstorming and often some solitude is needed for that. Do not start with the laptop and start rather having a walk alone thinking about your story :

1. You collect first all the ideas you can
2. You throw away all the ideas that don't support your main message
3. You keep the ideas that support the message
4. You connect them into a clear, concise story that talk about the main message

And ... by the way, when you have a great story, bad slides will not ruin the presentation but remenber that having great slides cannot improve a bad story( cf the slide above) When it comes to software topics it is the same : the great execution of the features of your software will not help you as far as you don't have a story that support your mean message !!!

To go further, have a look on slidesshare to the rethinking presentation design of Oliver Adria