Who: The first thing is to be sure your target audience knows that the web site is meant for them. The more specific the language that you use to help the target group identify, the stronger the identification will be, and the more likely that they will stay with you rather than surf on. "New for athletes" is good; "New for Runners" is better.
What: Clearly state what you’re offering, and again, the more specific the better. "Hydration Systems" is good; "Hydration Systems for Runners" is better (in fact, with the latter you've got the "Who" and "What" in one phrase).
Why: Now that they know it’s for them, why should they care? Get a clear and specific benefit into the pitch as soon as possible. "New" is good; "New design is easier to use" is better.
The mistake a lot of web sites make is trying to cast a wide net at first ("Athletes") figuring the more visitors the better, no matter who they are. This usually fails because there’s so much out there on the web that people will just cruise by anything that doesn’t scream “Hey, this is just for you!” Another common mistake is to skip right to the product pitches ("New hydration system") without the “who” and/or the “why”.
Having a clear Who, What and Why on your home page will go a long way to turn surf-bys into prospects.