The problem is that bitcoin tx fee depends on the tx size in KB.
To calculate the "correct fee", you will need to know the number of inputs and outputs of that tx, which is a bit too difficulty to those new users...
Yeah... Well, someone could make a calculator, which looks up the input addresses from the blockchain, and calculates the size and the fee?
There is still a problem in it, as people usually don't "consolidate" their bitcoin.
So, the sender may have used 20 different addresses to receive bitcoin before (say, 10 addresses for 10 different faucets, 1 for sig ad payment, 1 for selling domain, 8 change addresses, etc).
Well, they could type in all addresses they want to send from.
I am afraid It will be too difficulty to use then.