Don’t talk down to people. I think this should be obvious. But looking at the email offers I get, clearly it isn’t. Don’t tell people, “Not buying this product would be idiotic!” or “Only a fool doesn’t understand the value in this service.” This whole email marketing strategy relies on you making people understand the value they can get from you. And sometimes that means making people see the downsides of not buying. But you can do that in a friendly, respectful way.
Open and click-through rates (CTRs): Knowing who are engaged customers (those who open most emails and end up making purchases) versus inactive customers (who haven’t opened any emails in months) can be invaluable. Marketing campaigns announcing a new product should absolutely include those engaged customers, while re-engagement campaigns can be created to try and entice the inactive customers.
Are you willing to alienate people who disagree with your opinion? Sure, not everyone who disagrees with you will unsubscribe. But if you can’t take the risk of alienating a large portion of them, it’s better to avoid the topic. The only exception are friendly disagreements. For example, it can be a good idea to show your support for a specific sport team, even if you know many in your audience like another team. As long as they don’t take the sport very seriously, it can be just a fun thing to talk about. I could, for example, tell that I’m more of a dog person (we have two dogs) than a cat person. I have nothing against cats, but I like walking with dogs. I doubt almost any cat person will hold that against me.

Part of the problem is that people are confused about the difference between ‘strategy' and ‘tactics'. It's essential not to get these two confused. They are related – tactics are a vital part of what makes a strategy work – but they’re not the "be all and end all." Too many people neglect a full and comprehensive strategy in favour of a bunch of loosely-connected tactics. So, to recap: