Simple, their API. When you write a lot of code specifically for their API, you're somewhat stuck with them as re-doing all the work will take months. Also I'm international and most US brokers are not as friendly towards us. There is still a lot I like about IB, their platform (when it works) is great for me as are their tools. But since I pay quite a lot in commissions, I also expect decent execution - hung orders and mis-executions are a big no-no.
Set up an LLC. Look into Canadian brokers or Euro brokers that offer US markets. You can do better. IB doesn't really love you.
Why is an LLC important? I have one sitting around but why would that be a requirement? What alternative broker would you suggest? There doesn't seem to be many that tick so many requirements - low interest rate, DMA CFD, decent API, non-US friendly.