As others say, do lots of testing to figure out what's driving traffic where you want it.
But think of it this way: user attention is a finite resource. You need to figure out where it's directed a-priori and then direct it towards whatever will simultaneously engage them with your brand, satisfy their initial intent in coming to your site, and leave them feeling good about the interaction. If you put too much extraneous content directly in the user's face (Yahoo!, I'm looking at you...), you'll end up with frustration and lost or misdirected traffic. On the other hand, that will also be the result if the user can't find what he or she is looking for because it's buried in layers of navigation.
Your whole site should be streamlined: traffic in -> accomplish goal -> leave with satisfaction. Whenever they want to accomplish that goal again, you want them to think of how easy it was on your site, then come back to do it again.
Amazon and GrubHub/Seamless appear to have mastered this.