Assuming you're looking at the Open Source version of Magento... If you're coming from a smaller / simpler eCommerce platform, you might find that Magento requires more maintenance and upkeep. That's the #1 reason that I've found users can wind up dissatisfied. Magento doesn't cater to small businesses but is fantastic for mid-size and large organizations that can benefit from its flexibility and justify the resources to keep the site running like clockwork.
Best of luck!
