What's the least we could do, that would have all the necessary functionality to be shipped. In software/web industry you can change your product every day. Don't make the developing list be set for the next year or two.
Rest assure, the requirements for what's needed will change as you go along. Figure out what the necessary features are, and filter out the nice-to-have's, build quicker and deploy.

Text: Smidig 2008 [no] Picture: Modern Art

It’s the journey between pages or screens, not the pages and screens themselves, that can cause the most problems for users. Plus - problems with the journey are the most expensive problems to fix.

Design the journey between states first, before designing the states.

Reference: 101 things I learned in interaction design school

I do not spend all day working on projects, so I estimate in hours of work, not days.
I cannot do a project, so I need to break it into tasks and estimate the tasks.
I find it that when I estimate a task, I usually see it as just doing the work. Take in consideration communication, waiting and ad-hoc changes in requirements.
Have I done more or less exactly these tasks before?
Yes: Hey I'm a better estimator.
No: There's a larger chance of failure.

In hindsight I find this rule amusing: Take the first estimation of number of hours, and multiply that with π (pi), and you get quite close to actual time.

• Individuals and interactions over processes and tools
• Working software over comprehensive documentation
• Customer collaboration over contract negotiation
• Responding to change over following a plan

That is, while there is value in the items on the right, we value the items on the left more.

The only thing I miss is:

Purpose and Outcomes over Solutions and Features

Reference: Manifesto for Agile Software Development and Dan North's "Outcomes over Features – the fifth agile value?"

Speed up your development with not adding options to your code, apply sensible defaults instead. Ask yourself: What will 80% of my users be happy with? If you find an answer - cut the option - only add the option, if most people would ask/asks for it.
Example: Adding a drop-down to choose number of results per page? Set the default to 25 items per page. Sensible defaults also helps for consistency in your code.

Reference: Getting Real: It Just Doesn't Matter by 37 Signals