ASC, you're on the right track, but here's the deal.
The skin, or top-level UI, is called a launcher. You can easily replace it with choices from the Market.
However - the makers' UIs come with apps and widgets in addition to the launcher.
And you can't get the Google launcher from the Market, and even replacing things, you're stuck with the original stuff in the background sucking up storage.
To access the Google UI and/or free that storage, you need administrator access to the that microcomputer that is your phone.
Out of the box, the only one with admin access (we call it root) is your carrier - it's how they can do updates over the air.
To gain root access voids your warranty on most (not all) carriers and definitely requires know-how that intimates many.
And that burden, just to control your UI and limited storage, is the issue.
Hope this helps!