In his article Why Your Start-up Will Fail, Wil Schroter talks about lack of focus at the main reason.
Further he has some tips to keep in mind to help focus
* Perfect One Idea First
* Don’t Attack Too Many Problems At Once
* Identify The Real Opportunities
* Stick To What You’re Good At
Apart from these, it always helps to have a strong market focus. It’s very rare that a company succeeds on the technological merits (alone) of a product, unless it solves a real world problem. Getting Real has some good advice on building successful web apps.
Since startup is a emotionally (and financially) taxing, it’s very much essential to have the best team you can get.