I'm not much of an email guy, but the one newsletter that I really like comes from Jetsetter. The travel site's special focus is big, beautiful images: they can make a hotel room look so delicious, you want to lick the screen. I've been a fan of Jetsetter for years now, but I've never booked a trip through their site. Their content succeeded with me—it engaged and informed, and it built my appreciation of their brand—but it also failed to convert me into a paying customer.
Why is that? The answer is in the sales funnel.
Whether you're talking about the process of buying a car or the logic behind an infomercial pitch, there is a "sales funnel" that describes the stages customers go through on the way to making a purchase. Different people assign different names to different stages in that process, but by and large it all boils down to the following:
Awareness --> Desire --> Evaluation --> Commitment
First you need to know the product exists. Then you need to want it. Then you need to ask yourself whether you're making the right decision. Finally, you're all in: you make the purchase.
Jetsetter, like many other online marketers, loses the sale because they skip steps in the process. Jetsetter wants me to jump from liking the photo (awareness and the beginning of desire) straight into booking the trip (commitment). That leaves out evaluation, though, and I'm not going to spend thousands of dollars on a trip without first evaluating my options. On some level, Jetsetter is aware of this, and they try to compensate by offering limited-time offers ("sale expires in 5 hours") to force me to commitment. I'm sure that works with some customers, but not me.
What you need to understand is your customers will go through every stage in the sales funnel, whether or not you assist them in that process. If your site doesn't help with one of the stages, they'll go somewhere else to perform that stage; when they're done, maybe they'll come back to your site, and maybe they won't.
Respect the funnel, and give your customers the time they need to make a decision. If you try to hurry the process, you might just be driving them to a competitor.