I-O-U's - Debt and Credit with people
I have been using Mint, but not as much as I would like to. There is just one thing that keeps Mint from being PERFECT for me, and my primary money-managing tool.
My boyfriend and I live together, and a lot of the time one of us will pay the entire bill for our rent, and the other of us will "owe" their half. This also happens a LOT at the grocery store, with utilities, etc.
I find it VERY hard to budget things on mint, when I have to split things in my head, and keep a separate Excel spreadsheet wit my "Significant Other IOU account".
I recently looked at Splitwise, which is exactly the kind of integration I would love to see.
I never give feedback on these kinds of things, but I think this could make Mint so much more useful for so many people!