I don't think too many people who are in the position where four out of five of their own subscribers aren't even willing to open an email from them will imagine that they're doing email marketing too "successfully"?!

http://www.warriorforum.com/main-int...ml#post8661178

Open-rates depend mostly on continuity and expectation-setting.

In addition to the one linked-to above, these five threads, between them, cover many important ways of achieving and maintaining high open-rates and avoiding the things that lead to people having open-rates as low as 20% ...
What are the essential things to know about list building?
Lists: How Long to Presell - Averages
Website or squeeze page
Where to get reports to give away on opt in page?
Autoresponders vs. Broadcasts

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Thank you for your input

The bigger your list becomes, the lower your open rate will be, especially in the MMO niche.

It really depends on how you build your list and the initial emails. If you offer something of real value, the chances of people opening your mails in the future increase.

Split testing is the key, luckily there are some great resources available to see what works best. I suggest you check out products of Matt Bacak, he's the king of keeping data and creating products about it.

But that doesn't mean you should keep your own data too.