Unified and Threaded Mailbox
One of the major gripes with the existing iPhone platform is if you have many email accounts, you have to go in and out of each account separately to check them all. With iOS 4 you can you can now view all messages from all accounts in a single folder. Also, messages are arranged in conversations (i.e. threaded) so you can see in a single list all the messages from one contact.
However, you can't directly manage which email accounts are and aren't included in the unified mailbox or create extra ones. So, for instance, if you have multiple accounts for work and home you can't create a Work unified inbox and a Home one as well. Also, the unified inbox only shows messages sent to you so you have to go to the individual accounts to check on messages you've sent. So, while the iOS 4 has mostly caught up with the smartphone competition here, it certainly hasn't surpassed them.
Yes, crazy though it seems, Apple has brought its e-book reading software and e-book store (created for the iPad) to the iPhone and iPod Touch. Despite protestations that the screen is too small, the resolution too low, and the backlight of the screen causes eyestrain, Apple seems to think people will be willing to sit and peer at their tiny portable devices to read proper books. And, you know what, we can actually see the appeal. As with many things in life, if you make it easy enough people will give it a go and easy is certainly what Apple has made iBooks and the iBookstore.
For some reason you must download the app from the app store rather than it being included but beyond that, it's ludicrously simple to get under way. As on the iPad, books are presented on a virtual bookshelf and once your reading one, the interface has all the same beautiful page turning animations and easy indexing features. It is an annoyance that new books are fairly costly and the iBook platform is completely closed (so you can't transfer the files and view them on an ereader), and we certainly wouldn't want to sit for hours on end reading a full novel, but you can pick up out-of-copyright works for free and it's a great way of filling time while on a train or bus journey.
Improved Camera and Photos App
Upgrading to iOS 4 won't miraculously improve the image quality of the camera on your existing device but Apple has added a new feature: you can now digitally zoom. Just tap the screen above the shutter button and use the slider to adjust the zoom level up to 5x. As with all digital zooms, image quality takes a big hit as you're just making pixels bigger rather than physically zooming in to the object, and thus we rather fail to see the point. But, we can only assume some people have requested the feature so there it is. You can also now tap the screen to pick points of focus when shooting video on the 3GS.
The other image related update is an addition to the Photos app. You can now see your geotagged photos in a GoogleMaps style tab with pins showing where each photo was taken. Photos can also be 'tagged' with the your contacts and then you can search for a contact and it will show the photos they're in. What there isn't yet is social networking integration whereby your Facebook friends profile photos and contact info will appear on you phone, as there is in other smartphone platforms like WebOS and Android.