Design wise, MFP is a joke. I wouldn't give those bastards a cent, and I'd never use it without adblock.
-Can't delete a full meal or full day.
-No unit conversions (g/oz/etc).
-Need a plugin for something as simple as setting macro goals in g instead of the ridiculous 5% increments.
-After a certain time in the day, editing a future date keeps bringing you back to the current day. It seems to change around 8pm local time.
-"Copy from date" only works for past week.
-There is "quick add calories" but no quick add for individual macros.
-In the food list you can only chose between "all meals" and the current meal. But you can't easily look at foods from other meals. Overall the food list is a fucking joke.
-If you edit a future date, use the food list, and change "current meal" to "all meals" or vice versa and add the food, it adds it to the current date instead of the future date!
God, I could go on and on. The only thing that keeps me is inertia and their ok database (even though I still end up adding tons of my own foods because so few foods are listed by weight.)
/rant