Startup Principle: Applying the Constraint Equation Filter

With every person (consumer) in the world, there are two hard constraints to deal with. Time, and money.

If you're investing time or money into a business serving a B2C audience, you want to worry about the hard constraints, and the hard constraints alone. (Everything else is ops - solvable through smarts, process, technology and people).

Think about:
Q1) Does the product/service save a user's time?
Q2) Does the product/service save a user's money?

Then think about another two:
Q1) Does the target user have a time constraint?
Q2) Does the target user have a money constraint?

If there's a "Yes" and "Yes" on either the Q1s or Q2s, you've got a good starting point. 
The idea here is to be brutally honest on  the introspective questions, and not fall for the founder delusion (E.g. If you think your product is like the iPhone or Facebook).

Case Studies in the matter for reading: IKEA, Homejoy, Expedia

P.S.: Secret? Products that save a bit of time and lot of money, or, a lot of time and a bit of money. = Best positioned to succeed. (Practically all eCommerce is built on this rule.)
P.P.S: Luxury goods and entertainment are notable exceptions, of course.