Some things I noticed straight away:
New iPod Skin, New App Store Layout and Skin, New iTunes Store Layout and Skin. The top menu bar that shows your battery/3G/Time etc changes colour depending on the top menu of the app you're in. You can now Tweet from the Notifications Menu.
The new 3D maps look very realistic even the trees are 3D and it does at points look photo realistic. When you zoom in there is a lot of pixelation however. When you have hybrid map turned on which means you get places names, street routes + satellite maps and 3D the whole map becomes incredibly laggy. Sub 1FPS when panning and zooming. If you turn Hybrid off and turn just satellite on which means you get images + 3D then it's smooth with no lag. I think this is just a performance problem they will need to fix before release.
Everything seems quite stable I've not had any problems running it so far. I noticed inside the App Store that when you update apps it no longer asks for your iTunes password and it updates apps without ever leaving the Store. A nice feature I've wanted for ages. You can now also Launch apps straight from the App Store. I suppose this is for when you just buy an app and you want to launch it straight away.
Assortment of Pictures (Click any to view large):
