One of the biggest mistakes I see in business planning is people who think that "everyone" is their target market. For example:
I am launching a shoe company. Everyone wears shoes, thus my target market is all 300 million people in the US.
OK, come on. Who are you kidding? Your shoes are probably built for a certain TYPE of person. Figure that out, and then tell me who your target market is.
Read my post about this at Mommy CEO.