I'm been running a promo video animation studio for the last 4 years and worked with hundreds of startups.
The conventional formula for a 90 second demo video is as follows.
1. Problem -> 2. Solution -> 3. Call to action
Briefly explain what pain your product is solving.
This is where you explain why your solution is so awesome.
Explain how your product/services addresses and solves this problem.
List 2 or 3 benefits/features that make your solution special and better than anyone else's.
3. Call to action
Make sure you end the video telling people what to do next.
Either visit your website, download your app, or give you a phone call
For a 90 second video your script should be around 240 words or less.
Try not to sound too corporate and add a least one moment in the video can put a smile in your audience.
A successful demo video will be watched all the way to the end so make sure it's fun to watch.
I actually produced an online course where I explain the exact process of producing animated demo videos and it's now been taken by over 5000 people..
It's 10 hours long and covers script writing, storyboarding, illustration, animation and sound design.
You can check it out here: http://bit.ly/1eTlZ0Y
Please let me know if you have any other questions.