The installation procedure hasn't changed, but now it requires the two first CDs and if you want extra things (e.g. X11), you will need to have ready your third CD as well. I installed clean on both my Macs, because upgrading wouldn't work on my 12" Powerbook (Jaguar "had errors" the installation was insisting). That was not a big deal for me as I don't have any important data on my Macs, but it may be troublesome for people who who are trying to install on their primary machine. The default filesystem used is Journal-enabled (thank Dominic Giampaolo, filesystem god, of ex-SGI, ex-BeOS and ex-QNX fame, for that).
The biggest new feature on Mac OS X, according to Apple, is the new Metal-looking, multi-threaded Finder (re-written from scratch). The user now has handy shortcuts on the left side of any Finder window, shortcuts that lead to folders, other computers, media, disk images etc. You can also label files and folders according to their importance so you can easily spot them with a simple glance at the screen. Additionally, when double-clicking apps to load, a nice effect will zoom in and fade the icon's application, giving the user a smooth launch feedback feeling.
Exposé is indeed the coolest new trick we've seen lately on any OS, making the usage of virtual screens less needed (however, I will still keep my handy CodeTek VirtualDesktop utility). Using the keyboard+mouse, F9-F11 keys or "hot corners", you can trigger the OS to display your open windows in one large view. The effect is like a big bang, with each window shrinking in size and moving outward until they're all on one plane. You can then select which one you would like to re-appear on top. It's configurable to "expose" all windows, just applications, or just move everything out of the way for a clear view of the desktop. It would be great if Apple would add mouse gestures for Exposé though, because reaching for the F-keys all the time isn't convenient (and I dislike hot corners). The hot corners feature can result in accidentally engaging Exposé, which can be a jarring experience.
iChat AV is a useful AIM clone and if you happen to have an iSight (as I do), it makes it even cooler and more useful, as the iSight enables the videoconferenciing feature. Picture quality with iSight is marvelous, however I would have liked iMovie to recognize the camera as Firewire DV camera (which is what it is). Personally, most of the time I use Fire instead of iChat because I also need MSN, Y! and ICQ support. I hope Apple considers adding these popular formats to iChat (if that's possible, license-wise) as that way it will see many more "switchers" from Fire or Proteus to iChat.
Fast User Switching' is here too and it allows you to change users without logging out of your applications (by using another cool looking QuartzExtreme-powered special effect). This is a great little feature for families or people who share the same computer. I would have prefered the option to have a small 16x16 icon on the menu bar that clicking it would drop down the list of users, instead of displaying my full name on the menu bar. Even on 1280x1024, it does not leave enough screen space for some demanding apps that span across 12-14 menu items. It's bad enough for my name, but I imagine that someone with a full name like Alejandra Francesca Rodriguez López de Medina won't be happy at all to see that on her 1024x768 iBook or iMac... I filed a usability bug report with Apple a few months ago about this, and the reply was "this feature is working as intended". Right.