No, there is more to a distro than them sharing a common package manager/format. The kernel, which is heavily patched by canonical, differs from what Debian ships. Canonical also has a bad habit of carrying a lot of non-upstream patches for other core system components. Ubuntu may have been derived from Debian at one point, but they are fairly different at this point. So much so that you cannot take a system running Debian and 'upgrade' (or downgrade, depending on your point of view) it to one running Ubuntu.
That doesn't say much, as sometimes ubuntu and debian upgrades are far from smooth.
As a long Debian & Ubuntu user, I believe you're exagerating the work invested by Ubuntu to custonize Debian. Betond the default desktop environment and attitude regarding proprietary drivers, the work done by Ubuntu is rather negligible.