Nick, at risk of glossing over a great many variables, in my experience it usually comes down to how well the message (your content) is conveyed to the market (your list).

Some of the variables are: suitability of offer; appropriateness of language; layout, legibility and graphics; whether the message is an unadorned sales pitch or an offer for valuable information, etc.

Overall, it comes down to the value you are perceived to provide. The answer to that is slightly more complex, but common mistakes are: 1. providing solutions to non-existent problems; 2. giving answers to questions that have not yet been asked; 3. misjudging the importance of your offer. 

Sometimes it's possible get it exactly right. For example, last year I ran a highly targeted (single shot) email campaign to a specific audience of several thousand, selling a seminar. I sold out four seminars to 85 attendees in a day and a half, with a waiting list for the next one.

The offer was right, the timing was right, the list was right, the perceived value was right. I think you have to keep working on your style and content (perhaps seek professional advice) until you find the right balance.

I used to send out a fortnightly "Love Letter" to the audience of my online magazine, Thelma . 

Through doing this, we ended up with loads of brand evangelists who would share the email with their social media audiences, and who would reply and tell us how great they thought the emails were and how much they looked forward to them, and encouraged others to sign up.

The thing was, that we didn't try to sell them anything, or even try to get them to click through to our own product's site. 

We simply shared images, anecdotes, stories etc of things that were relevant and interesting to our audience. And thought we didn't sell anything, we built our own audience and reputation in that field, by building up "credibility" and the vibe that we know all about the kind of content and things that our audience are into and want to know about.

So, I'd say that email campaigns can be hugely successful, in terms of building your brand identity and building a relationship with your audience, and turning them into evangelists for what you do.

But probably less so if you want to use it solely to sell directly.

A company that does a great example of this is Native Digital who do a brilliant weekly email called "Native Insights" that features links to articles and information, and quotes from around the web, that are relative and inspiring to their audience and clients. I'm pretty sure they get a great response rate from these. Sign up to check it out, I love it.

From my experience Nick, as Steve mentioned it does depend on many variables.  I'm not too sure of the protocol here on SavvySME, but hopefully it's OK to send you to an article I wrote which includes some very good tips. Hope it's helpful to you. 

How to Write Killer Emails that Capture your Audience

I agree with Kealey about creating an engaged audience for your brand vs a direct sales approach.

Nick hi, sure content is important but if your list quality is sub standard you are always going to be fighting a losing battle. I'd initially focus on your approaches to acquiring list members, where quality trumps quantity, so look to recruit based on a sound proposition which your content will honour across the future relationship. In the seo arena think about developing quality white papers which are available on sign up to your newsletters. Finally on the subject of newsletters, they can become a liability, as one feels compelled to issue the monthly communication. My thoughts would be only publish when you have quality content, not when the newsletter publication date comes around.

If you know that the customer is King, then the content must be Queen?

If you want to turn your email subscribers into long term customers, then you need to feed them what they want. Why not ask them first?

not enough, something like 13% of emails i used to send out in email campaigns got opened by the recipients, clicks even less, leads maybe 1% and purchase you can imagine how many....

not a good way to promote

Frequency & Content are the two big keys. If the communication is issued too frequent and has little relevant content subscribers will reduce. It's a fine line between tell your clients what they want to hear and sharing with them what's important to your business. Always review the content from the recipients perspective, add value and your list will steadily grow.