It's the old KISS metaphor (Keep It Simple, Stupid). Email has an attention-capturing window of opportunity that is greatly diminishing. Some say three seconds, some five, but either way, it isn't a lot of time. Nielson Norman Group produced a newsletter usability report in June of 2006 ("Email Newsletter Usability") which indicated an average newsletter has the reader for up to 40-50 seconds, while a marketing or promotional email retains the reader for less than 5 seconds.
The fact is we read less, scan more, and make decisions based on where we are drawn into the message--some through a contextual hero image, some through flow of images, typography and layout, and some by modularity. You should understand the basic principles: a simple call to action, buttons, text links and image roll-overs make quick comprehension easier. If it doesn't pass the scan test, then it won't be compelling.
I recommend you test your design on an internal focus group. Flash the email in front of them for five seconds and have them tell you what it said and what the call to action was. If they can't tell you, then you should consider revising.
Go back to the principles of design and use contrasting colors, but do so for the right reasons: to draw the eye, reinforce a value statement, and amplify the call to action. In addition, you have another consideration - how your colors appear within the email inbox interface. Do your light blue borders get muted out in AOLs predominately blue interface? Cool design can get blurred when there is an animated image of an eBay IT campaign flashing at the bottom. Is there a competition of cohesive?
While the email industry has migrated to a concept of design in which the top 200-300 pixels are a virtual banner, too many designs have disproportionate layouts (almost like an hourglass). Your email should flow smoothly and be evenly distributed if your intent is for the reader to flow through content. Eye tracking studies show how most users scan e-mail and apply those logics (if you want more information on this, check out http://www.eyetools.com.) If the intent is to design a singular message, then design it to a five-second preview. That way the eye is conditioned to the flow and not tempted to roam.
4) Message focus.
Email is direct response, not a website. Infuse what you know about good media and banner design into your creative by minimizing your real estate. This will cause you to be more concise in your messaging and creative treatments. Just because you have a never-ending scroll doesn't mean you need to use it all. Use imagery to quickly communicate a message, not merely for beautification. While I love the retail industry, the cataloger view of delivering email messages (with the large postcard-like image) has shown diminishing response. Catalogers are continually amazed when simple SALE messages, without that large postcard image, result in a boost in sales. Never forget that because this is a sales message, a response is required.