In today's fast-paced society, you only get 5 seconds to sell your stuff. Forget the 30 second elevator pitch, you're down to 5 seconds. This article has some good ideas about how to make the most of your 5 seconds when advertising on billboards, TV/radio, direct mail and email:

http://smallbusiness.foxbusiness.com/marketing-sales/2011/03/09/second-rule/#

What the article doesn't mention is it's applicable to your website as well. Most people choose to click to a website (opposed to being presented an ad or commercial,) and testing has shown that you actually get a little more for a website: about 7 seconds. Remember though, this includes load time! Make sure you home page loads quickly, and make sure your message is prominent and easy to understand. If a visitor can't easily identify what they're looking at and confirm that home page is where they want to be and that site has what they're looking for, you'll lose them.