When thinking about how to protect something its useful to think about who you are protecting from. This normally starts with brainstorming categories of attackers along with their capabilities and motivations. From there you often move on to defining persona’s for the attackers that help personalize and more granularly categorize their skills, resources and motivations.
This is just a quick stab at a Mind Map of who potential attackers are for a Bitcoin business, you may also be interested in the post I just did on Bitcoin itself. Both are far from complete but they might be useful to you when thinking about how to to protect your systems and prioritize appropriately.