Trivial in the sense of googling "how to install KDE on Ubuntu", picking a result that looks somewhat recent, and following those steps. It ends up being a handful terminal commands which shouldn't be too hard for anybody who has used a keyboard before. That's how I did it at least. There might be more UI centric options.

Also, trying to chase the elusive 'casual user' is what caused all the GNOME UX mess in the first place I guess. I'm not an 'archetypical' Linux nerd, I hate wasting time with fixing stuff that should "just work", but I'm also expecting a computer to be a professional tool which I can customize to my needs (within reason at least).