The philosophy of a web start up company is extremely important, and I think this should be set before you even start coding. I believe in lean, agile, pragmatic approach to business and this is reflected in the core philosophies of the venture:

Lean and mean, bootstrapping
No lavish offices, no expensive staff, everyone works remotely and keeps costs to a minimum.

Agile
Keep steering the ship, fast and agile. Less documentation and more prototypes, less guessing and more testing.

Revenue from day one
If there is no revenue strategy, or business model then it is not a business.

Features first scaling later
Think about the killer features that your customers want and need, worry about scaling later, prove the concept first.

Release early and quickly
Your product will never be perfect and it will never be ready, so just accept that and release. As soon as it is being used by real customers you will have to change it anyway.

Involve customers before the product is built
As developers we are always guessing what our customers want, so why not ask them. I like to build a wireframe of the site and involve customers when testing it.

Guerilla marketing
Clear brand message, be known as the best in one area, and customers will come to you.

Sound business principles
Prudent financial strategy, at least six months operating cash in the bank before profits are taken from the business.

Focus on businesses not consumers
Several web apps I have built have focused on end consumers and trying to get them to pay is almost impossible. Businesses are much more likely to pay for services if you can add value to them or save them money.

Fun and excitement
Make sure you are passionate about what you are building, and that you have fun working on it.

Part time work
This might be a contentious one, but until the business is making a profit then work on it in your spare time. 10-15 hours a week is enough time to build a web app in a few months.