It has to be their support chat function and their useful Help articles. I did not get into my business so that I could run mail marketing software, so I appreciate anything that helps ease the learning curve. Email marketing is a necessary evil, and Mailchimp lessens the unpleasantness considerably. With no prior experience, we got up and running quickly. (We tried a few of these apps, and this one seemed the easiest.) It's complicated running email campaigns and newsletters, but once we mastered a few basics, I was very happy with the results. And it scales up as you grow. When our mailing list grew, it did not become exponentially harder to use. They offer a lot of bells and whistles that we don't need/use, but our emails look good and are easy to compose, adding graphics, etc. It automates a lot of repetitive tasks for you -- like your footer and address, etc. And we've never had a problem getting reported as spam.
And this may seem minor, but I like the whimsical nature of the occasional chimp graphics. Don't get me wrong -- this is work, not play, but it's nice to see a little chimp every once in a while when you complete certain tasks. Business is hard, so a little levity along with the functionality is welcome.
The price! It gets expensive quickly. I guess the upside to that is it makes you hone your marketing so you are profitable. You have to cut out waste and hone your mailing list so that it's more efficient. And from my research, the other programs cost about the same. I don't mind paying for ease-of-use, but I wish they would explain "cleaning" and "archiving" and other functions that would help me get my monthly costs down. I'm not planning on jumping ship anytime soon, but if they raise their prices again (they just raised them recently), then I will be gone.