One of the big questions remains whether it will be in the same/similar iPhone 4 casing or in a redesigned, bigger screen, all new casing. TiPb has independently heard that we shouldn't get our hopes up for a redesign this year, but with Apple we won't know for sure until we see it.
Assistant sounds like the killer feature for iPhone 4S/iPhone 5, with the ability to handle natural voice input and hold a conversation to parse complex tasks like setting up appointments, informing invitees, finding friends, etc. (Unfortunately, it still can't actualize "Tea. Early Grey. Hot." but it's getting close...)
Check out Mark's complete write up for a lot more details, linked below.