Being in this industry it is important that we feel the pulse of the market. Therefore I guess everyone would read the news sites every day. This makes some of the content in the newsletter a bit out of date. But having said that, I guess a substantial % of affiliates would love to read the newsletters religiously.
I think you need to pack in everything that you have mentioned in the newsletter with special focus on what Alex has suggested if you want to make it sticky. Each week your newsletter can have
1) A few suggestions/ tips to make more money or add value to the affiliates’ business.
2) A Case study
3) Success stories of affiliates
4) Success stories of player promotions
5) Non-Gaming opportunities
6) A promotion for readers, the prize can be cash or premium links or adwords vouchers (this would get you more regular readers, I guess this is your primary objective)
Hope it helps.