How do you handle chargebacks? What about fraud? If you don't properly protect your users, you're not really providing a good service, and you need to make the rules incredibly clear for your merchants, they aren't going to work with you.
How are merchants supposed to respond to the volatility of bitcoin? For a currency that sees semi-wild (compared to standard FOREX) swings in value, they could be waffling between the red and the black on a daily basis. There's no value in holding bitcoins when there are better currencies out there.
Where do you make money? Transactionally? On a flat-fee basis? In loaning out the cash? The problem with banking is that the fractional reserve system has created a risk environment such that you can't make money unless you're leveraging your reserves for lending, and that's an incredibly complex system on its own. The big banks make money because they are vastly huge. How would you move into that scene? Is the intent to be acquired? How do you leverage startup inertia to compete against a company doing hundreds or thousands of times the volume in a day than you'd hope to see in a month?
Just a few questions. Many people have given this particular problem a lot of thought, and none of them have been able to make it work as of yet.