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.