It's sort of like comparing the then-current Windows 98 Second Edition and today's Windows 7: You wouldn't doubt for a moment that the newer version is much more polished than the earlier edition, but you'd be able to get around in both operating systems and get work done.
What's far more striking than the differences between Corel's early and crude KDE interface and Ubuntu's slick GNOME 2.28 front end is the abundance of finished applications. For example, for practical purposes, your only choices back in 1999 for Linux Web browsers were Netscape or a terminal-based browser like Lynx. Today, you have your pick of Firefox, Chrome, Opera ... heck, thanks to Wine, you can even run Internet Explorer on Linux, if you really had to.
Though major software vendors like Adobe haven't brought over flagship programs like Photoshop to Linux, they have brought over those applications that are used almost daily by most users such as Adobe Flash.
In addition, open-source software has gotten a lot slicker than it was ten years ago. For all practical day-to-day use purposes, there aren't many options in between the open-source OpenOffice 3.1 from the proprietary Office 2007.
If you look deeper than the desktop and its applications, you'll also see that 2009's Linux has another big advantage over its predecessor: hardware support. Ten years ago, you had to make darn sure that your PC hardware could support any given Linux distribution. Today, modern Linux distributions support the vast majority of both internal PC components and peripherals. I can't recall the last time I had any trouble installing or running Linux on any modern PC. For more on that, check out Linux-Drivers.org.
True, sometimes devices aren't as well supported as we'd like. It seems to me though that is no longer just a Linux problem. I ran into endless trouble with device support in Windows Vista, and things don't seem that much better with Windows 7.
Of course, there are other major changes as well, but most of those have to do with overall advances in technology. Wi-Fi was just becoming standardized in 1999 with the introduction of 802.11b. No operating system of the day came with Wi-Fi support baked in. Now we take being able to connect from almost anywhere as a matter of course.
And, of course, today you can buy PCs with Linux pre-installed on them from vendors like Dell and System76. While I suspect most people still download and install their own Linux distribution, you no longer have to do it.
Still, though today's Linux desktops are bigger, faster, and have far more software and hardware support than the Corel desktop, or its 1999 counterparts from Stormix and Caldera, you can see that their family tree is rooted in this first Linux desktop aimed at the mass market.