Android’s data monitor is topnotch, but not too user friendly for non-techies.
I think Windows Phone 8 is trying to solve that with Data Sense. The only problem with Data Sense though is that it has to be supported by the carriers.
Tracking which apps use your data is pretty simple. Just install My Data Manager (Free) and allow it to track the data your apps use. At any time, you can open up Data Manager and take a look at how many megabytes (or gigabytes) any app is using. If anything is sucking your data plan dry, make sure that app isn’t running in the background (or just delete it from your phone altogether). Solving the problem is easy so long as you know which app is using up all of your data.