I, so far, haven't seen anything RAM/processor intensive from the list. Stuff like Cortana staying active all the time may not be possible for a 620 (battery killer) so we will have to activate it physically maybe.
Nobody would Cortana use all the time, especially in public (where I spend most of my day), and some people may not even use it at all.
The new snapdragon processors, as far as I know l, have special low power chips for assistants that keep it always on. Those which don't will have to activate it physically (much like bing tellme).
My battery life didn't change with amber or black. Must be something else wrong! And battery won't be better cos so many new features need more power I guess?
I think you will miss the ability to share your screen wirelessly. Some devices are excluded and I think 620 is one of them, but you will probably be able to share your screen through usb.