One of this issues I experience with my current banks and credit cards is that I do not get a feeling for how much I am being charged or credited for interest. The rate can change and I am probably informed via a long letter of the new rate, and at the end of the month/year I am surprised the amount is either much more or much less than I estimated.
As Mondo will be providing an overdraft feature in the future and hopefully interest to accounts in credit, it would be really good show transparency of these charges - If I am maxing out my overdraft and paying £4 per day in interest, I would like a push notification each day the charges to make me do something about it. Similarly, If I am in credit and raking in the interest I want to know each day that my coffee was for free to encourage me not to spend so much.
The option to set your own frequency of interest transactions would be great.