I'd have to say easy to use, precise and customizable would easily make for an awesome app! Maybe with a reminder too could be handy! Right now I'm using myfitnesspal and I know the reminder would be nice and so would be the ability to have more than 3 meals plus snacks...
Good things it does have though is meal memory! That way when you make the same snack or meal often you don't have to go hunting the whole list of ingredients... It also has the handy feature of scanning an item if you want instead of searching for it. And last handy feature I can think of right now is the calorie counter which includes how many calories you burn through cardio(though it doesn't take in consideration the effects of hiit or the tabata protocol etc and lacks a count for weights though I'm not sure what formula could be used to count those calories spent)