ubuntu-desktop is a metapackage. It is empty, and only contains metadata such as dependencies and a description, to provide a simple way to install a whole desktop environment. It will also make it possible for to "push" new packages to be installed. This can be very much helpful over the five years Ubuntu is supported when the development of an application has stopped and they want to provide an alternative by default.
gnome-power-manager is a hard dependency of the
ubuntu-desktop package, you can't simply remove it without breaking the
ubuntu-desktop package installation. And this is what APT is telling you here: it can only proceed with your decision to break the metapackage requirements. You can proceed with removing both packages, but that will make upgrades a bit harder, as you will have to handle any new packages yourself.
$ apt-cache show ubuntu-desktop
Depends: alsa-base, alsa-utils, [...], gnome-power-manager, [...]
Description-en: The Ubuntu desktop system
This package depends on all of the packages in the Ubuntu desktop system
It is also used to help ensure proper upgrades, so it is recommended that
it not be removed.
In case you want a very minimal Ubuntu system with only the packages you really pick yourself, then:
Whenever installing new package, be sure to omit the recommended (but not required) packages, by using an
sudo apt-get install --no-install-recommends packagename
or when using
sudo aptitude install --without-recommends packagename
This significantly minimizes the amount of packages installed as the result of the single package selected. However, not all packages have separated hard dependencies and recommended ones separated in the requirements.
ubuntu-minimal. You'll have to install Ubuntu using the Server CD and specify you want a minimal install by pressing F4 on the first screen (see below).
Then after installation, you'll have a non-GUI installation. Install the
ubuntu-desktop desktop without the recommended packages (see above for how) for a minimal Unity installation, or consider a more minimal GUI installation with LXDE:
lubuntu-core. Don't worry about having installed using the Server CD. The Ubuntu flavour is just a set of packages.