What is the Unmet Need your are Fulfilling?
It's very important that you start with more than just some cool technology. Your product or service needs to fill a need. If you're using a 1 page Lean Canvas, that's what goes in the "Problem" section.
So what is the unmet need your are filling? Is it really "unmet"? If it's a need that is already being addressed in the market, why do you think this is a startup opportunity?
What is Creating the Need?
Startups usually address a new need -- or they leverage a new technology to create a new and better solution for an existing need. If you're addressing a pre-existing need with pre-existing technology, then why hasn't someone already delivered that solution, and what makes you think this is a startup opportunity?
Is the Need Compelling?
A startup is an unproven company with an unproven product. And let's face it, your first product is likely to have some problems. To have a chance of selling your product (and possibly attracting investment money), the need you are addressing has to be compelling. It should be a "must have", not a "nice to have".
Who has the Pain?
If you're solving a problem, it's a problem that is causing someone pain. Who, out in the market, has the pain that you will address? Is there a way to quantify that pain for a typical individual? Across the entire market?
Who is your Customer?
If we define "customer" as the one who pays you, who is your customer? Is your customer different from you "user"? (For example, if you're building a website and you expect revenue from ads, then your customers will be advertisers -- not your site's users.)