What is the normalized transaction hash of a transaction?
It is a hash of the transaction without the input scripts.  SIGHASH_ALL says all inputs and outputs are included in the hash.  These are the areas covered by the standard payment methods and they cannot be modified without invalidating the transaction.  One of the problems, though, is that there are other payment methods which do change the inputs and thus the normalized transaction ID would change in those cases.  But it does cover the common transactions.