Last, but not least, you need to make sure you're in compliance with each affiliate program's terms of service. Some programs will allow you to promote via email marketing, others, like Amazon, do not. Keep a detailed spreadsheet of all the affiliate programs you belong to and what methods of promotion are allowed. Not following the rules could get you banned from a program — which means you've lost a source of revenue.
But I think the biggest deciding factor in this, goes back to the site as a whole and all of the other posts. Are the genuine? Is the blogger constantly trying to push products? I’d like to think I’ve been doing this long enough that my audience knows I’m not out to make a quick buck – and I think even relatively new bloggers can prove this based on their other content.