So, all four upgrades completed, with absolutely no problems or hitches (at least none that have manifest themselves so far). Now, I'm fairly competent at running my computers, but I wouldn't class myself as a serious "under-the-hood" mechanic. That's to say, I'm happy enough to mess around with config files, set up hardware and all that, but I don't have the sophisticated understanding that would allow me to appreciate subtle changes between Ubuntu releases. That being said, presumably there must be some evidence of an upgrade?
Gnome - this is my preferred desktop environment, and with Compiz effects is quite flashy enough for me. (I've always found KDE a little garish for my tastes. I did give KDE4 a whirl when it was released, but it seemed a bit flaky at that time. I'll try it again in the coming months as I hear it's a bit more stable now, and it's quite possible that it offers a fresh new way of using the desktop.) Gnome seems to be characterised by small incremental changes - here we have version 2.26, which on the faceof it seems pretty much the same as before. You can read about the main changes at the gnome website.
Probably the most significant change for me will be improvements to Evolution's integration with MS Exchange servers. As is probably the case with many, my workplace is pretty much Microsoft focussed, with email handled through an Exchange server - if this does what it says on the tin, it'll help things.
Apparently Gnome offers better support for second monitors and projectors. That will be most welcome - when I tried setting my main notebook for projection, it seriously arsed up my xorg configuration. Since then, I've been using a shell script to switch modes using xrandr.
Ubuntu 9.04 has a new notification system, which seems to be graphically quite pleasing, but it's not clear whether it represents any advance other than aesthetics.
Network Manager seems to reconnect to my wireless router when resuming from hibernation a bit more enthusiastically than in did in 8.10.