A new feature which everybody was waiting for and which Apple decided to implement, is multi-tasking. Or at least, this is what Apple says he has implemented. It won't actually be a full multi-tasking. True, applications like those ones used for music will be making multi-tasking, but games, for example, won't. As Steve Jobs showed in the presentation, if you play a game and you want to....browse a website for example, you have to double-click the home button (the game will pause), go to safari, browse, double-click again, and go to the game, and resume playing. So, the game won't be actually making a real multi-tasking.
Apple said that they didn't implemented multi-tasking since now because it consumes more resources and power, and they have found a method which resolves this. The main advantage of this "multi-tasking" is that apps like Skype can now run in the background, so you don't have anymore to stay into Skype to speak with somebody, without having any possibility to do anything else.
Now, iPhone can finally use folders. You can now put apps for games or for weather together, for example, in a singe folder, so you don't need anymore to browse through more screens just to find an application. This is something good.
Also, you can now set wallpapers. I don't know why Apple didn't implement this earlier, but it's good they finally made it. And you now have "unified" mailbox, so all the mails will come at the same time from all your accounts. This has bad and good points, and in my point of view, I think that it's better without this feature.
What really sucks, is that Apple is going to implement iAd. This means that now everything on the iPhone, iPad and iPod touch is going to have ads. Steve Jobs said that this is a good feature, and it is, for advertisers, but for regular users, it is a disaster.
And, from what is known, there will still be no Flash in iPhone OS 4.0. This is totally stupid. I now think that the iPhone is never going to have Flash.
Apple says that this upgrade will be available only for future generations of iPhone, and for the 3GS. Guys who are working on jailbreak solutions showed clearly that Apple does this just for the people to buy new iPhones. Because they ran the OS 4.0 beta on both 3G and even 2G, without any difficulty.
And, you have to know that this new software was jailbroken in less than 24 hours since it was launched, so, probably, when the new OS will be available for anybody, jailbreaking will be easily done on any iPhone.