Make it shareable. Send content that people want to share, and make it easy for them to do it. Sure, subscribers can forward your campaign to friends, but that's a lot to ask. Include a public link to the web version of your campaign so people can read it outside of their email programs, and consider adding Twitter and Facebook links to your newsletter, so readers can share your content where they're already active. When their friends start sharing and subscribing, you'll know it's working.
The ideal type of opinion (unrelated to your expertise) is one that doesn’t offend anyone, but makes some people relate to you more. For example, I’m a vegetarian. I used to protest whenever there wasn’t a big piece of meat on my plate. But when I tried eating just veggies for a couple of months, I realized I felt a lot more energetic. Sure, it’s more ecological. And often animals are treated cruelly. But those aren’t the things I emphasize if someone asks why I’m a vegetarian. No one in their right mind has a strong negative reaction to someone eating food that makes them feel energetic, right? But for some vegetarians it’s a very meaningful choice, so it might have a positive impact on those people. That said, I highly doubt that my choice of diet have any direct impact on my sales.
When you're using email as a marketing tool, you're working on a pretty personal basis. This isn't ‘drive-by' stuff -you're directly targeting customers from their inboxes. There's a huge amount of potential which comes with this kind of marketing... but you'll only reap that potential if you REALLY understand both the marketplace you're operating from, and the audience you're talking to.