KDE-Forum.org

That depends on how your KDE3 desktop looked like.
You can use the folder view as desktop instead of the plasma desktop, you can ban all plasmoids from the desktop (I'd keep the panel), you can use the alternative classic menu (which is a plasmoid), you can have different wallpapers for your desktops when you bind them to activities (but you cannot show those wallpapers in the pager), you can use any other window decoration, style or icon theme - but it must match the KDE4 requirements, so you maybe have to rename a lot of icons in your favorite theme if it is not availabe for KDE4.