No turn by turn because it's integrated with Siri, something we've known they're not going to bring to the 4.
No Facetime over 3G because the 4 has different models for different networks and they focused only on the 4S and above since they're 'universal phones.'
Why is this so hard to understand?
There is nothing about Siri that has to be there for the turn by turn to work. Simply disable all of the rest of Siri's features but keep turn by turn. It would actually be REALLY simple to do, and nobody would associate turn by turn on their iPhone 4 as "AAAAMMMGGGG I haz Siri now!"
I am not sure what you are saying about different models for different networks the 3G facetime (or how that is relevant, rather) but I am quite sure that has nothing to do with it, especially since we can easily get facetime over 3G from a third party. It has nothing to do with hardware, it is 100% software driven.
I always like to ask how people would feel if their desktop OS just happened to have features removed for their older modal machine. Maybe OSX lion shouldn't allow the new notifications on the core2duo macs and only allow them on the core i5/7 macs? Or maybe Sandy Bridge chips should be exempt as well since they are now on the third gen ivy bridge chip? It would be extraordinarily easy to implement the limitation, by the way. But people would be outraged. Why? Because, frankly, it is a ridiculous thought.