The arrival of multi-tasking on the iPhone marks a major departure for Apple - this feature has been in the offing since the launch of the 3G - and improves the iPhone's ability to compete with other platforms in the future. In fact I think this will go a long way to killing off a large chunk of the competition, as iPhone-holdouts who needed multitasking see that barrier removed.
The arrival of social gaming on the platform looks to rival the Xbox integration which Microsoft announced with Windows Phone 7, whilst iAds targets Google's AdMob offering for Android users.
That seems to be the pattern for iPhone OS4, covering the improvements that newer platforms have made/addressing the iPhone's shortcomings when compared with other platforms and in general keeping Apple up with the game. I suspect the next move will be to address some of the iPhone's hardware shortcomings - new camera, screen, processor - with a hardware refresh around mid-summer.