Good overview of the announcement from Apple last friday on iPhone OS 4 features. Scroll down quite a bit to get to the meat of the detail.
The most significant thing is full multitasking and app swapping. Very interesting to see how seemless it feels and although usability wise it's very simple it appears there is significant engineering innovation here. Especially if the drain on the battery is minimised. The APIs seem very straight forward and not only for the iPhone but for the iPad too.
The folder idea is excellent - one of my personal gripes about the iPhone is how hard it is to group like applications - so I'd glad to see this included.
Unified mail and multiple exchange accounts is good - finally be able to get all my calendars from my multiple hoster all in one place.
Probably one not to underate will be the social gamming network. This will bring some very very interesting possibilities!
iAd is interesting and an excellent platform - but they really need some better ad formats. Like Admixa :) !