Well, my "day job" is designing mobile and online banking apps, so I've spent some time thinking about this question (a couple of years, in fact).
The most important thing I've learned (through various iterations, user tests, and multiple releases) is this: Mobile banking is a specialized focused application. It's shouldn't be a one-to-one mapping of all the features available in online banking. It's about making the core things as easy as possible: balance checking, quick payments and transfers, and transaction lists/review. You get that core stuff right, you're half-way to an outstanding mobile banking experience.
Now, if you get the core features obvious and usable, how do you get to the "next level"? The first thing to do is to understand the context and frequency of use and how people think of mobile banking on their phone. This should help you take a first cut at the must-have "value added" features versus "nice-to-have" or unnecessary features.
One quick example: When I was on a major mobile banking app design project last year we decided to not implement the "add a payee" feature for international payments. The reason was simple: After surveying users we identified that this was so low on thier priority list that it was easy to drop (analytical data also told us it was a seldom-used feature in the online system). We had a design solution to allow users to add international payees, but it was not optimal and would have been a significant development effort. Deciding what NOT to do is as important as focusing on what features you are going to provide.
When it comes to identifying "value added" features, look at functionality that will save users time and hopefully money. Check deposit using the camera on a smartphone is a HUGE delighter for customers, as are "pro-active" alerts that warn when something bad is about to happen (like an overdraft). Again, conceptualize ideas and present them to users to get an early sense of what will make your app outstanding.
Finally, fully embrace the interactive aspect of mobile apps… use pagination, animation, and other techniques to make the app respond and "live" for users. Just don't go crazy with stuff flying around everywhere – interactivity should support user tasks, not distract them from what they are trying to do.