One thing Debian and Ubuntu share in common is the use of Debian package management and the tools that go with it. At a terminal level, apt is in use for package management.
Where things start to differ is with GUI package management, Ubuntu uses the Ubuntu Software Center by default and Debian uses Synaptic. For experienced Linux enthusiasts, this is actually preferred because Synaptic is a better software tool than the Software Center. However, newbies might find themselves missing the Ubuntu Software Center as Synaptic lacks the polish found with Ubuntu's default option.
The other side of the coin is Ubuntu's PPAs and Debian's backports. The idea with Ubuntu's PPAs is that you can keep up to date with the latest individual software titles of your choice. With Debian, users can have the same experience using Debian backports. Basically, these are software titles from Debian testing made available for Debian stable users. With both Ubuntu and Debian, user need to beware, as bleeding-edge software can sometimes introduce new problems. My advice is to avoid either approach, unless there is a bug fix or a feature you're gaining by using the bleeding-edge version of any given software title. While older titles might seem a bit boring, all too often, they're more stable.
One other area where Ubuntu and Debian differ is that most networking in Ubuntu just works out of the box. And in the event that a proprietary driver is needed, the restricted driver management tool will handle this with ease. By contrast, Debian isn't going to run many wireless devices out of the box. Granted, the needed blobs are available from the Debian repositories; however, it's up to the end user to install these blobs and enable many common wireless chipsets to work with Debian.
A parting example of putting in the work with Debian would be installing proprietary video drivers. To an experienced user, the Debian way is quite straight forward. Simply browse to the appropriate page, add the needed repo, then paste in the install command for your driver. Ubuntu users however, have been spoiled (to a fault) by the proprietary driver manager. The real comedy is that in truth, the Debian way is actually faster than the Ubuntu method. Debian users only need copy and paste in the contents of two command boxes from the wiki. On the other hand, Ubuntu users depend on the GUI, which has been known to be far from perfect. Worse, if something goes wrong, there's no verbose reaction to look to without diving into the logs themselves.
The point here is Debian requires more hands-on user experience. Some folks will prefer this, while others may balk at the very idea of making Linux on the desktop a deeper experience.
So is Debian a solid alternative to Ubuntu? Yes, if you're willing to adjust your expectations.
For those users willing to learn how Linux works and who have no problem with spending a little extra time adding in needed repositories for additional hardware support, Debian is a fantastic option. Easier to use than Arch Linux, Debian still provides its users with a speedy, stable desktop you can rely on for many years to come.