Free, as in freedom from proprietary
Perhaps one of the nicest features in Debian is the ability to have both GNOME and KDE on your system at once. The two even integrate into each other's menu systems. It looks a bit odd, but if you want everything and the kitchen sink, Debian makes it easy.
Another area where Debian differs from its downstream offspring Ubuntu is its emphasis on free software, as in freedom. Debian removes the binary blobs from the default Linux kernel and, while Debian so far has stopped short of removing all non-free software from its repositories, none of its default desktop configurations include any non-free software. Instead you'll find free software alternatives, like Gnash instead of Adobe Flash.
If you don't mind the occasional proprietary software or driver, you can of course grab most of them from the repositories.
What you won't find in Debian is the extra layer of polish that the Ubuntu team puts into their releases. The Debian 6 desktop did get a new theme, but it's a far cry from Ubuntu's obsession with perfecting the gradients in toolbars or continually tweaking its icons. If you want that level of obsession, but don't want to give in to Steve Jobs, then Ubuntu is for you. If you really don't care how many pixels are which shade of grey, then Debian works just fine.
If this release of Debian has a drawback, it's that the hardware requirements have been bumped up slightly and when it comes to downloading the latest copy, and Ubuntu users might be left scratching their heads - do you really need eight DVDs to install Squeeze? No, you don't. All you need is the first CD or DVD and a network connection to grab any extra packages you might want to install. The extra CDs and DVDs are just extra packages, most of which you probably don't need.
While Debian is delivered as an all-in-one package (there's no Kubuntu or the like), this release does see some new pre-configured setups that Debian refers to as Debian Pure Blends. There are Pure Blends tailored for a multimedia workstation, specialized GIS map workstations, chemistry workstations and several others.
Perhaps the oddest news in this release is Debian's decision to offer a variant with the FreeBSD kernel stuffed into the Debian userspace. Yes, you read that right, Debian without the Linux. The FreeBSD version of Debian is, for now, what the developers call a technology preview with "limited advanced desktop features." That means, if you've been clamoring for a way to relive the Debian of 1999, this is probably for you.
For the rest of us, the Debian of 2011 makes a nice, stable alternative to Ubuntu, even if it does perhaps lack a little of the shine that has endeared Ubuntu to the masses. ®