I too have been using iOS4 on the 3GS for most of the beta and now GM release. The Multitasking will be nice once most of the common apps like Pandora are updated to take advantage of the true multitasking API's.
The retina display is key for me as well as the HD recording, I can say the camera is much faster now at snapping and focusing shots than the 3GS even was, which is nice. I only wished the shutter button was not based near the bottom of the camera app, its cumbersome to hold with one hand and still touch that almost unreachable button at times. They should make it so that a tap tap on the screen or tap and hold to take a snapshot and single tap is to focus.
The zoom is also nice, but being a digital zoom, it will tend to pixelate any shot you have that would otherwise be crisp and sharp with native focus. But I still can't get over the speed difference, I have missed so many shots in the past with the older OS, fumbling about waiting for the iris/shutter to open in the app.
There are plenty of other new items not talked about in the Keynote, like when sending photos being able to reduce at that time by compressing for smaller file size etc.
The new Jawbone headset works great when paired between two iPhones and can simultaneously allow audio feed from one of the phones while still being tethered to both phones for receiving or making calls.
The tethering, not sure a whole lot of people are going to take advantage of this, but it will be perfect for those who were thinking of carrying another MiFi device, now the iPhone can be both a mobile device and MiFi for new iPad WiFi users for example.
So Many things I've run across using iOS4 on the 3GS in daily operations, that I tend to forget just how resourceful Apple was in implementing them, they are very key yet seamlessly integrate into the OS to the point you forget they are now there.
I can't wait to play with my new Digital Assistant, My personal iPhone4
I am jealous however that my shiny new iPad doesn't have this Retina display, but soon enough I'm sure.