The most important feature that was introduced today by Steve Jobs is "Multi-task".  This feature is already available on other smartphones, but was missing on the iPhone, iPhone 4.0 completed that feature.

New features in iPhone OS 4 include:

* Folders - you can create folders that include other icons on the homescreen
* Enhanced email, including unified inbox, threaded messaging and multiple Exchange accounts
* Game Center, which is some sort of a social gaming network
* Enhanced enterprise capabilities
* iAd - a new mobile advertising solution. Apple will sell and host the ads, 60% of the revenues go to developers. Ads will "keep you in your app."
* iBooks

Source: unwiredview