We've come a long way from Ubuntu 4.10. It's easy to forget the great improvements to the Ubuntu desktop we've experienced over the years. Every once in a while we should count our blessings.
Update Manager. Warty didn't have an update manager to perform updates and version upgrades. All we had was Synaptic, or apt-get for the console user. Update Manager greatly simplified the process for a lot of users.
Ubiquity. Before Ubiquity, Ubuntu used the same console-based installer as Debian. That's not a bad thing, though. It was a solid installer. But things could be better and faster, and Ubiquity was the answer.
Ubuntu font. This one's pretty recent. Fonts seem such a small thing, but clearly a desktop's appearance is greatly affected by its font. The Ubuntu font is open and beautiful.
Software Center. While Update Manager simplified updates for the Ubuntu desktop, installing new software was still a problem for new users. Even long time users can get stumped. Open source software tends to have really strange names that don't reflect their functionality. Synaptic wasn't suitable. So, Software Center was born. It gets better each iteration.
Unity Desktop. This one's up for contention. There's no doubt Unity is making waves, but with many users not liking Unity it's hard to say if Unity will improve the desktop experience or if Ubuntu should switch to polishing Gnome Shell instead. It's seems like a 50/50 split.
There's more, of course. Application indicators, overlay scrollbars, and notify OSD all serve to enhance the user experience in small but significant ways.