What is the main problem it solves?
How does it solve that problem? Can you explain this in 1 sentence? 1 paragraph? 1 page?
Who is the main audience?
Who are the secondary audiences? For example, moderators, admin users, customer support.
For example, admin tools, reports, automated maintenance?
For example, batteries, cables, WiFi access?
Does the CMS need to be updated?
Will data become stale and need to be replaced or updated? For example, zip codes change frequently.
Are there APIs that will be upgraded and require changes?
Are there social plugins or other third party tools that may change or stop working? For example, Google Maps, Facebook Likes, etc.
Will the storage eventually fill up?
Will the database explode?
Will cost of some small component (email, text messages, database storage, etc) scale very quickly if successful?
For example, if a site will recommend restaurants, where will the list of restaurants come from? Where does the information that powers the site come from?
If from other users, when, how and why? Nobody wants to build a database for someone else.
For example, if my site is about showing me my friends activity, but none of my friends are on the site, what do I see that makes me want to invite my friends?
Before something goes viral, it needs to find its first 100, 1000, and 10,000 users.
Do the math!
Signing up 10,000 users in 3 months requires that you sign up 100 people a day.
Signing up just 100,000 users over the first year is about 275 people per day.
How many people do you have to reach per day to get 275 to sign up? If you can convert at a respectable 3%, you need about 9500 people per day to have the chance to say yes. But converting 3% is really high! So you probably need 20,000 people per day to get 275 to sign up.
Paid advertising costs $1+ per click.