Mac OS X 10.4, first hours
I couldn't install Tiger out of the box due to a problem in the disk, fsck reported more than 50 Overlapped extent allocation (file \d+). Lucky me, some guy documented what that means and how to fix it in single user mode. After that the installation was smooth.
There are infinite changes in Tiger, for infinite greater than 200. Some changes are cosmetic (as the merge of title and toolbars in some applications), some are major upgrades (as Xcode 2, or QuickTime 7, or Preview 3), some are important new additions (as Spotlight, or Dashboard, or Automator), and some are removals (as Stuffit). Tiger is the first Mac OS X where Java 1.5 runs, you can get it separately and it does not interfere with the installed 1.4.2.
I have to explore almost everything yet, for instance I have not touched Automator at all yet (I am very curious though about how they did approach the design of such a tool), but playing around with this new toy I noticed Safari RSS is fast, damn fast, in addition to have a cool interface to RSS feeds. The integration of Spotlight in the system is such a powerful feature that I am sure I can't foresee its secondary effects yet. One of the clear applications are smart folders (which are virtual, live folders defined by a query) in the Finder and in Mail.app, also upgraded. Mail.app by the way has been refined but has a new toolbar nobody seems to like.
I expected the result set in Spotlight searches to be more immediate than it is. Sure, they come up very quickly, but not immediately as I'd expect from an index and the demos in Apple conferences. I wondered for instance whether I could launch applications with Spotlight instead of with Quicksilver, but at least in this G4 that's not practical. Maybe that's dramatically improved in G5s.
Dashboard is an interesting addition. As people develop widgets that are helpful for me to have there, I'll see whether I integrate Dashboard in my desktop usage. I don't monitor flights so often :-), but the dictionary and the calendar are already useful for instance.