The installation routines seem to have greatly improved with Meerkat, especially the way it starts the download and then asks some of the personal questions. How this relates to this topic is:
Why not just add a little diag box into the process that asks the user which GUI they would like to install: GNOME, Unity, KDE, Enlightenment etc... and show a nice little downsized screenshot of each alternative as they tick the radio boxes?
It would be quite a bit of work for the developers, I realise, but not impossible. Naturally, click on "Forward" goes with the default, and it would be too much work to include everything, but the main contenders: GNOME, KDE, Unity, can't be too hard to setup the script files for.