Re: Can 12.04 be lightened?
NO NO NO! ubuntu-desktop is the metapackage. This installs the whole thing and you may as well install Ubuntu outright.
It is always harder to remove what you don't want from a full-fledged install. This is because APT is designed very conservatively to leave all sorts of libs/services/modules untouched when apps are removed: in the interest of safety, to preserve dependencies and so as not to inadvertently break other apps. This is why those of us interested in highly efficient and minimal installs always start with next to nothing and then build up only what we want.
You will also find that Ubuntu is simply a beast. It cannot be helped. What you like about it--presumably all the fancy effects, Unity, Compiz, etc etc--is what makes it such a pig. Some people claim that they can get Unity 2-D running as fast as Xubuntu or Lubuntu, but I've never been able to. Even without Compiz/3D/spinning desktop cubes, etc, everything else that comes with Ubuntu slows my older machines to a crawl.
If you want to go the subtraction route anyway, then you can't use anything more than 12.04. This is the last version that comes with Unity 2-D. Anything later only has 3-D. This will limit your upgrade path, but since 12.04 is supported until 2017, this isn't really a big deal. To invoke Unity in 2-D, just make sure this is the option selected at login.
Post your specs. It really is impossible to advise you without knowing the details.
Newb: How far must I jump to clear the ledge halfway down?
Guru: It's bad to jump off cliffs. Let's look at better options.
Newb: Stop harping about "best practices" and just let me jump.