Eu não acho que seja possível responder totalmente a esta pergunta, pois ela pode ser muito ampla ou baseada em opiniões, mas o As distribuições do Debian Wiki on Children Software fornecem uma boa visão geral dos objetivos do Ubuntu:
Ubuntu
Ubuntu is a Linux distribution that starts with the breadth of Debian and adds regular releases (every six months), a clear focus on the user and usability (it should Just Work, TM) and a commitment to security updates with nine months of support for every release. Ubuntu ships with the latest Unity and KDE releases as well as a selection of server and desktop software that makes for a comfortable desktop experience off a single installation CD.
O Manifesto Debian de 1994 dá a você uma idéia do que Debian como um projeto tenta realizar:
A.1 What is Debian Linux?
Debian Linux is a brand-new kind of Linux distribution. Rather than being developed by one isolated individual or group, as other distributions of Linux have been developed in the past, Debian is being developed openly in the spirit of Linux and GNU. The primary purpose of the Debian project is to finally create a distribution that lives up to the Linux name. Debian is being carefully and conscientiously put together and will be maintained and supported with similar care.
It is also an attempt to create a non-commercial distribution that will be able to effectively compete in the commercial market. It will eventually be distributed by The Free Software Foundation on CD-ROM, and The Debian Linux Association will offer the distribution on floppy disk and tape along with printed manuals, technical support and other end-user essentials. All of the above will be available at little more than cost, and the excess will be put toward further development of free software for all users. Such distribution is essential to the success of the Linux operating system in the commercial market, and it must be done by organizations in a position to successfully advance and advocate free software without the pressure of profits or returns.
Então, basicamente, o Ubuntu se configura como uma distribuição fácil de usar, com interfaces de usuário sólidas e padrões sãos. O Debian foca em suporte sólido e espalha a influência do Linux nas configurações profissionais e do usuário final.