7 is kind of an arbitrary number and the use of the word "top" seems a little dubious. Is there an 8th reason that just didn't make the cut. In any case, the site is called top7business.com and I suppose it focuses the discussion. This list cites consumers but we think that it applies to BtoB as well. The fact of the matter is that email campaigns are the same as any kind of advertising: you have to grab attention and hold it and not focus on price.
From the post:

Confusing content. Successful ads don't try to tell everything the advertiser knows in one ad. The best ads are focused around one or two main ideas.
Too much emphasis on price. Most ads are written as if price were the only variable. Consumers always want a good, fair price but have no way of knowing if they got the best price. Consumers don't respond to ads because of price alone.

Unrealistic expectations. Expecting major league results in a time frame that's too short with a budget that's too small. Make sure expectations match up with budget and timing.

Too little frequency, repetition, consistency. Most advertisers stop short because they run out of patience and/or money just at the time when the campaign is about to start working. If you have a good ad, keep using it.

Too much about the product, not enough about the consumer. The most successful ads are about the consumer...why they should buy the product and how they benefit from using the product. Most ads waste too much time and space talking about the product and not enough about the consumer and their needs.

Not memorable. Not meaningful. Not sticky. Consumers can act only on the ads they remember. Most ads we encounter are irrelevant. The best ads break through the mental filters by talking our language, touching our emotions and creating imagery that resonates with the consumer.

Failure to understand and focus on the consumer. Consumers don't have a lot of patience for ads that talk only about the business or the product. No one's listening. No one cares. Speak to consumers about what they're interested in and they will respond.