One risk that can scare off investors is the presence of gatekeepers in critical parts of the supply chain. By "gatekeeper" I'm referring to a person or company that can prevent your business from obtaining access to resources that your business is dependent on. If you need another party's permission to obtain access to key resources (e.g., distribution), your company faces the risk that it will not have access to those resources (the gatekeeper might say 'no') or that it will be charged a heavy toll in order to access them.
One classic example of gatekeepers existed in the mobile industry in the days before Apple opened up the iPhone platform to developers. In 2007 and before, many entrepreneurs creating applications for mobile phones needed the permission of Verizon, AT&T or a few others in order to launch their companies. Without the approval of these giants many of these entrepreneurs couldn't deliver their products to consumers. Those that received permission paid a high price for access to their mobile networks since Verizon and AT&T had significant negotiating leverage.
It's worth noting that concentration in the tier of the supply chain that has the gatekeepers generally increases risk. When there are fewer gatekeepers it's less likely that entrepreneurs will get the deal that they want, as 1) the entrepreneur simply has fewer chances to close the deal and 2) gatekeepers that face less competition are less motivated to take risks and new businesses or new ideas.
Fragmented tiers of a supply chain can also create gatekeeper problems for entrepreneurs. If competitive pressures are limited, gatekeepers may not explore new opportunities and may negotiate aggressively on pricing and terms. The gatekeeper issue in un-fragmented markets can be especially problematic for companies that threaten the status quo of gatekeeper's marketplace.
The gatekeeper issue is one that investors pay a decent bit of attention to. There are, however, ways to mitigate this. The first way is to cut out the partners that you might have been dependent upon. If your gatekeepers played a key role in distribution, sales or marketing, this might mean selling your product directly. For example, in the days of the closed mobile platform some companies sought to distribute their product over the mobile web, alleviating their need for partnerships with Verizon and AT&T. The other, slightly more obvious way, is to secure gatekeeper partnerships early-on before you invest much time in your company or seek to raise capital.
The more your company depends on partnerships the more calculating you should be thinking about how to mitigate the gatekeeper risk for your business.